Why twin tails?
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Brian
Dear Sir- Hyperbole and flight test are joined at the Hip. My comments paraphrase the Fly off results accurately. My understanding of the knock on the F-18 had to do with its resemblance to prior A/C (F-5) and the F-16's superiority in agility and response.
The strakes themselves create higher lift by planting their vortex on the wing root, preventing earlier Stall, especially at high AoA.They improve longitudinal stability by supporting the Forebody (nose) and prevent it from wallowing, again at high angles of attack. I haven't found your direct linkage re: strake to VS.
This is from memory, like I said before, I don't use Wiki, and if I am confident, I'll write it. I think those who run to "the book" insult the respect of their counterpart, as well as their own. I stand to be corrected at all times, and welcome wholeheartedly new information that improves my knowledge. Both A/C make my Heart go thump, they are both wicked cool, this isn't about whose mates fly what.
The strakes themselves create higher lift by planting their vortex on the wing root, preventing earlier Stall, especially at high AoA.They improve longitudinal stability by supporting the Forebody (nose) and prevent it from wallowing, again at high angles of attack. I haven't found your direct linkage re: strake to VS.
This is from memory, like I said before, I don't use Wiki, and if I am confident, I'll write it. I think those who run to "the book" insult the respect of their counterpart, as well as their own. I stand to be corrected at all times, and welcome wholeheartedly new information that improves my knowledge. Both A/C make my Heart go thump, they are both wicked cool, this isn't about whose mates fly what.
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FEHoppy
In general, I agree. End plates, strakes, VG, Fencing, etc. Shepherding Airflow (Winglet) creates benefits in lessening drag, increasing manouverability, and reducing Stall speed or increasing control effect. "Ducting" airflow can get extremely complex. Intake efficiency, washout, "Flex-Wing"; it remains a bad idea to add anything to an A/C that won't, (can't) perform. There are limits, mine begin with the appearance of the first equation. I admit I am mathaverse. If I understand something, I don't need to teach it or prove it. "Coanda Effect", Blown Wing, Vectored thrust?
OK, pull chox.
Brian Abraham- By Ballistic, I mean Energy Assisted (Created) "Flight"
Watching an F-18 Waddle down the Runway, CLEAN, at 40 knots and AoA in the neighborhood of 70 degrees, at an Altitude of 50 feet with both pipes roaring and burning two parallel ruts into the runway isn't strictly speaking Aerodynamic Flight. Neither is an F-15 at Mach 1 and vertical, its wings producing Zero Lift (Unloaded). On the Eagle in that config., those wings aren't wings at all, more like Fins, (As in Rocket Fins). Ballistic. If I'm wrong, ok, I'm listening.
Airfoil
OK, pull chox.
Brian Abraham- By Ballistic, I mean Energy Assisted (Created) "Flight"
Watching an F-18 Waddle down the Runway, CLEAN, at 40 knots and AoA in the neighborhood of 70 degrees, at an Altitude of 50 feet with both pipes roaring and burning two parallel ruts into the runway isn't strictly speaking Aerodynamic Flight. Neither is an F-15 at Mach 1 and vertical, its wings producing Zero Lift (Unloaded). On the Eagle in that config., those wings aren't wings at all, more like Fins, (As in Rocket Fins). Ballistic. If I'm wrong, ok, I'm listening.
Airfoil
Joined: Mar 2008
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From: USA
Hoppy,
In a much simpler response to your post, yes, you are correct that an H-tail increases elevator effectiveness through endplate effect. However, the size of the horizontal is rarely a driving factor in selecting this design (the wings are always much longer, right?).
However, endplate effect is one of the factors in using a T-tail. It allows for a shorter vertical, and if the tail is swept, a smaller horizontal as well since it places the aerodynamic center of the horizontal farther away from the CG.
And I'm not sure who brought up the F-16's twin tailed flyoff competitor, but it's also of interest to note that the YF-17 was a highly modified F-5 that eventually became the F-18 after some navalization (for lack of a better word).
In a much simpler response to your post, yes, you are correct that an H-tail increases elevator effectiveness through endplate effect. However, the size of the horizontal is rarely a driving factor in selecting this design (the wings are always much longer, right?).
However, endplate effect is one of the factors in using a T-tail. It allows for a shorter vertical, and if the tail is swept, a smaller horizontal as well since it places the aerodynamic center of the horizontal farther away from the CG.
And I'm not sure who brought up the F-16's twin tailed flyoff competitor, but it's also of interest to note that the YF-17 was a highly modified F-5 that eventually became the F-18 after some navalization (for lack of a better word).
Joined: Sep 2002
Posts: 2,188
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From: La Belle Province
One reason for having twin vertical tails on a supersonic fighter (not yet mentioned I believe) relates to the way the shocks behave and the interference effects between the fins.
Basically, at subsonic Mach numbers the two fins interfere with each other, in the manner of a biplane's two wings, which reduces their effectiveness as lifting surfaces (and thus their directional effectiveness). At a suitably supersonic speed, however, the two fins essentially move out of each others' direct interference, and begin to work like two truly independent surfaces.
It so happens that this kind of effect - where the directional stabilising effect gets more pronounced at higher/supersonic speeds - is just what the doctor ordered to overcome the increasing directional instability one finds due to the increasing effectiveness of the forward fuselage.
The alternative is to design a truly massive single fin sized for the high Mach number case and then cope with the excessive directional stability at low Mach numbers by other means.
Basically, at subsonic Mach numbers the two fins interfere with each other, in the manner of a biplane's two wings, which reduces their effectiveness as lifting surfaces (and thus their directional effectiveness). At a suitably supersonic speed, however, the two fins essentially move out of each others' direct interference, and begin to work like two truly independent surfaces.
It so happens that this kind of effect - where the directional stabilising effect gets more pronounced at higher/supersonic speeds - is just what the doctor ordered to overcome the increasing directional instability one finds due to the increasing effectiveness of the forward fuselage.
The alternative is to design a truly massive single fin sized for the high Mach number case and then cope with the excessive directional stability at low Mach numbers by other means.
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Sir
That's not necessary. What's in the short pipeline are articulating strakes.
Your suggestion that the forebody, in gaining stability at high Mach introduces instability aft is a good point. At high speed the A/C can swap ends unless the Tail continues to anchor the aft and the front. The new strakes will be retractable above Mach 1. The A/C test bed for this feature, along with flex-foil is the F-18, none other. Why are the tails canted on the F-18? Mainly for two reasons. One, the "interference" you identify. The directional surfaces are in each others "ground effect" unitl a certain speed, and the effect leaves the foil from top down, relieving the thinner portion at the tip of very high load due to flutter. Second, each surface provides angular lift due to its departure from the vertical, similar to a winglet in that regard. Also, the Rudders can both be turned in (pigeon-toed), providing down force aft to increase rotational authority off the CAT. The F-18 is a treasure of aerodynamic demonstration, and will continue to be for a good long while.
Airfoil
Your suggestion that the forebody, in gaining stability at high Mach introduces instability aft is a good point. At high speed the A/C can swap ends unless the Tail continues to anchor the aft and the front. The new strakes will be retractable above Mach 1. The A/C test bed for this feature, along with flex-foil is the F-18, none other. Why are the tails canted on the F-18? Mainly for two reasons. One, the "interference" you identify. The directional surfaces are in each others "ground effect" unitl a certain speed, and the effect leaves the foil from top down, relieving the thinner portion at the tip of very high load due to flutter. Second, each surface provides angular lift due to its departure from the vertical, similar to a winglet in that regard. Also, the Rudders can both be turned in (pigeon-toed), providing down force aft to increase rotational authority off the CAT. The F-18 is a treasure of aerodynamic demonstration, and will continue to be for a good long while.
Airfoil
