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Old 06-27-2009, 01:10 PM
rmks2000 rmks2000 is offline
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Tail Types

OK. This is not a build thread, but a question about aircraft tails. Could someone help explain why stabilizers are flat, dihedral (F-86), or anhedral (F-4)? And the same goes for craft with multiple rudders (ex. F-15 vs F-18 vs SR-71). Also, what about those without the traditional appendage (Beechcraft Bonanza)?. I realize that it has to do with stability, but what are the benefits of the different variations?
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Old 06-27-2009, 01:29 PM
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Don't know about the horizontal stabs why some have dihedral, anhedral ect, but as far as vertical stabs an aircraft needs a certain amount of verticla tail area to keep it stable. You could put a single larger stab in place of 2 smaller ones but in military applications especially 2 smaller ones makes for a smaller radar cross section (RCS) ie stealth. If you have 2 vertical stabs then you generally put rudders into both of them. I'm sure there might be an example somewhere of a plane that had 2 vertical stabs but only one rudder but I'm not aware of it.

Regarding the Bonanza and its V-tail it's an attempt to reduce weight and drag. Theoretically you need less surface area if you combine the horizontal and vertical stabs hence less structural weight (weight is the enemy of everything that flies) and less surface area to create drag.

If I remember right from my reading even though theroretically that's the way it's supposed to be it has generally been found that the ruddervators need to be larger in practice than theory would indicate thus negating the benefits and becoming mostly a fashion statement. Except in the aforementioned case of reducing radar cross section.
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Old 06-27-2009, 01:34 PM
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Tim Crowe Tim Crowe is offline
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Multiple rudders on jets tends be a requirement for extra rudder input in the event of single engine failure. And to a lesser extent redundancy of controls in case of battle damage.

Though I can't help thinking in a modern military jet at full power would need the live engine throttling back a bit, in order for the rudders to work properly.

I remember talking to an RAF pilot who flew Lightings (English Electric) Low down, depending on which engine went you either bought the farm or kissed the angels.

The Bonanza had a V-tail. This helped reduce drag and weight. But did give the tendancy for the aircraft to be a little bit unstable in the yaw axis. I think the V-tail was dropped in the late 80's - Looked nice though!

Sometimes multiple rudders was just down to style or pratical reasons - You don't need such a tall hanger.

I'll leave the anhedral/dihedral bit for others to answer - I've not a clue!

Tim
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Old 06-27-2009, 01:51 PM
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From what I remember from university, anhedral was used to keep the horiz stabilizer away from flutter due to engine exhaust, for example the Vickers Viscount and Comet. This was particular so at high angles of attack. The vortex in the case of stall would in such a case render the tailplane useless with catastrophic results, as demonstated by the initial models of the BAC 1-11. They solved that by installing a stick pusher, so that you would not enter a state of stall at all.

Dihedral in wings normally relates to achieve sufficient high rates of roll as required by the certification design rules, but what that would do for tailplanes is beyond me. The only aircraft I recal with such tailplanes is the F-4 Phantom. Anyone with better or more light on this?
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Old 06-27-2009, 01:53 PM
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OK, I'll give this one a shot.

First off the Bonanza is the easiest to explain. The V-tail has reduced drag compared to the conventional tail. The trade off is some additional complexity (and weight) in control mechanisms and structure. The Bonanza retained the V tail as a styling affectation, while the Debonaire and T-34 otherwise similar aircraft, have a conventional tail.

Twin tails have multiple explanations. In early prop twins there was the belief that having a rudder in the slipstream of each engine increased rudder authority and allowed for the reduction on total area. Conveniently it also allowed a greater field of fire for rear facing gunners.
On the F-14 and F-18, and many of the other carrier aircaft, multiple tails allows for a reduction in height for hangar deck clearance, while maintaining the required amount of area for proper control. It also allows for redundancy of control in the event of combat damage. This is especially true of the A-10, whose rudders also mask the engines IR signature from most angles.

In genral anhedral decreases stability of a lifting surface, while dihedral increases it. Draw an arrow at 90deg to the surface and see how dihedral consolidates the forces, and anhedral scatters them.

Tail surface in a conventional aircraft, however, often lift "down" to balance other forces, so the 'anhedral" on an F-4 is actually dihedral from the point of view of the tail surface.



Any help at all?
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Old 06-27-2009, 03:18 PM
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In the old days, the style of tail sometimes had to do with other than aeronautical engineering considerations. For example, the original Douglas DC-4 had a triple tail because, with the nose gear elevating the aft end of the airplane, a single tail with sufficient surface to provide adequate directional stability would have made the airplane too tall to fit inside most airport hangars.

And, of course, the low-aspect ratio (short and fat) wing of the Short Stirling bomber likewise was a result of the width of Royal Air Force hangars of the time.

Don
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Old 06-27-2009, 04:21 PM
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Plastic Bonsai Plastic Bonsai is offline
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Tail surfaces are usually have anhedral or dihedral to position them in the best area to provide stability which is influenced by the downwash from the mainplanes. If you can get away with it the T-tail arrangement is very efficient aerodynamically as it gives good stability in the pitch axis, clear of the body drag wake and it increases the fin effectiveness by acting as an end plate - making it seem bigger to the airflow. The usual downside is that you run the risk of the tailplane being blanked by the wind wake at high angle of attack making stall recovery problematic. Other factors concerning tailplane positioning can be the trim changes with flap changes and blanking from airbrakes.

There's a lot of estimation and wind tunnel work etc on an aircraft configuration that doesn't always work out as expected when they actually fly the aircraft. At that point things get interesting as a variety of tricks are attempted to get around the problems.

Last edited by Plastic Bonsai; 06-27-2009 at 04:22 PM. Reason: Spelling
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Old 06-27-2009, 04:36 PM
rmks2000 rmks2000 is offline
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Thanks for all the great input Guys!
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Old 06-30-2009, 01:51 PM
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As i recall the unusual features on the F4 including the utterly strange tailfin were all done to patch up the aerodynamics which was so bad when it was first designed...
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Old 06-30-2009, 06:45 PM
Zathros Zathros is offline
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The Beechcraft Bonanza (Doctor Killer) ruddervators had a tendency to flutter, which caused a few tail sections to fall off. Pilots were told to keep their feet on the rudder pedals to act as shock absorbers ( you should always have your feet on the rudder pedals!). I worked on a few of these planes ( I used to work under an A & P who was also an A.I.) and managed to do quite a bit of work on small aircraft. I had a few chances to fly in the Bonanzas but they had a very narrow and tricker loading criteria which made them not to forgiving. I'll take a Piper Lancer anyday over a Bonanza. Mooney also had a strange rudder which was slanted foward. These planes had a high rates of accidents but it was mostly due to the wing cross section which was a laminar flow style and could stall at full speed. You really had to cut back into the wind to get the air flowing again, sometimes as much as 10 degees, but what a fun plane to fly. All my knowledge is based on working on the airplanes and flying with the pilots of these planes and may not match "offical" data.
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