Tail Types

[shrike]

And the swept tails on later Cessna products (150, 152, 172 et c) are solely products of the marketing guys to make them look 'modern' and 'space age'. Of course it reduced the effectiveness of the surface, theus they had to be made larger as well.

[Golden Bear]

Originally Posted by Zathros

The Beechcraft Bonanza (Doctor Killer) ruddervators ...

Yeah, I was also going to point out the nickname. Classicly the tailplanes allow you to go back and forth in a flat plane (rudder) and roll (elevators). The elevators also assist the climb but the history of development shows that they had their biggest impact in allowing an actual turn (rather than a flat turn).

The twin tail thing of recent jets has been dealt with except for the comment of "the Russians did it so we want one too!" itis that led to the F-15. Dihedral on the wings helps with lift during a turn since one wing winds up more or less parallel to the ground. Descending during a turn is an issue as one can imagine. However, high performance aircraft were, since about WWII timeframe, built to be on an edge for stability, etc., which meant that near zero dihedral was necessary for best performance with high powerplant aircraft. There are MANY other factors that come into consideration for this including wing cross section, fuselage shape, etc.

For the "Doctor Killer" I don't know the actual history but you would need to actively tweak both rudders since you couldn't actually yaw independently of roll. Seems difficult to me.


Carl

[Zathros]

I have to admit when I saw the name of this thread (Tail Types) I thought it was about something else, my bad.

[rmks2000]

Zathros

Sorry for misleading you :D

[Retired_for_now]

Without the math - an aircraft's configuration of surfaces is determined to provide the desired stability inflight.

Basics - the center of gravity (balance point - the airplane twists around this point) must be ahead of the center(s) of pressure (all the "pushes" on the airplane go throught this point) for static stability - rocket with fins at back flies straight, fins on the front tumbles. For a controlled aircraft, the CG is usually pretty close to the CP and you use the control surfaces keep the pointy end out front. The kicker is you have to look at it from all angles.

From above (looking at pitch, nose up/down) the center of lift from the wing is usually about 1/3 of the way back from the leading edge. The center of gravity should stay just ahead of this point. If you think about it, the weight of the airplane is now in front of the lift holding it up - so you'd expect it to nose down and dive. That's where the tail surface comes in. The horizontal stabilizer is usually set so it pushes the tail down in normal flight - holding the nose up. The elevators (or the whole surface with a fully flying tail like most modern fighters) pitch up or down to change this force, lifting the tail to dive or pushing down harder to pitch up and climb. Having a tail continuously pushing down causes extra drag - which is why some aircraft (Burt Rutan's, Beech Starship, etc.) put the "tail" out front as a canard. Instead of pushing down on the tail they lift the nose to fly level - creating less drag because they are producing useful lift (which offloads the main wing's need to produce lift and lift always produces some induced drag).

From the side, you see the vertical tail sticking up for the same reason, it moves the center of pressure for crossflowing air (which you'd get if the airplane goes a bit sideways - yaw that is) aft of the center of gravity. The rudder is there not to turn the airplane, but to push the tail around to keep the airplane aligned with the airflow (technical detail, in a turn the outside wing produces more lift than the inside wing, lift makes drag, so the airplane tries to slide it's nose to the outside of the turn - slows the turn and makes drag so you use the rudder to keep it lined up). An airplane with a rudder that sticks up above the fuselage also tends to roll the airplane - true balance would require equal sized rudders on the top and bottom of the tail. We don't do this because the bottom one hits the ground, though you see more symmetrical rudders on some large airplanes like the C-119. Not normally a problem since you shouldn't be using the rudder by itself anyway (unless you are trying to "pick up a wing" with it).

From the front things look better, but we're now concerned with the roll axis. For a mid-wing airplane, a perfectly square arrangement of wings/etc. would give you a situation where the airplane doesn't care how it flies, tends to drop a wing and spiral into the ground. Positive dihedral (wing tips higher than wing roots) does two things: it shifts the center of gravity lower, hopefully making a pendulum effect and keeping the bottom side down; it also shifts the direction of lift from each wing - when upright both wing's lift is pointed above the fuselage (lift acts perpendicular to the wing), when rolled the lower, flatter wing produces more useful lift since all of its lift points up while the higher, angled wing wastes some of its lift pointing sideways, so the airplane lifts the low wing and comes back level. Negative dihedral reverses this effect and is usually used to move things around to correct some other defect (engineering constraint?). A high wing airplane (Cub, Cessna, Catalina, etc.) usually does not need dihedral since it stays level due to the pendulum effect of the CG well below the wing (and a straight wing is stronger).

Dihedral of an aircraft tail is usually done to fix something else - the forces from the main wing overcome those from the tail and twisting the airplane by its tailfeathers is usually a bad thing. The original Bonanza is a special case - the rudder-vators need to be V'd because they're half vertical tail and half horizontal tail (originally designed to reduce drag since there's only two rather than three pieces - and because they could and it looked cool). You also move the tail around to get it in good air - T-tail on Lockheed bug-suckers and Boeing's 727 keep the horizontal tail out of the main wing's turbulence during a stall -allow you to recover the airplane. A couple of little Pipers used it as well.

Twin (or more) tails usually reflect a need to get more rudder surface (control power and stability) without making one great big rudder. Little one's are also usually structurally stronger and stiffer than sticking one 20 foot tall tail on a little airplane. They are also used as add-ons to counteract other mods - you'll see them a lot on float planes, added to provide more stability to cope with the big pontoons hanging out front.

Twin tails can also be used to shield hot exhaust from IR missile shots from the side (A-10).

You see a lot of slanted tails on stealth aircraft (SR-71, F-117, F-22) to keep from pointing a flat reflector back at a radar in normal flight.

Swept wings (or tails) are just for looks unless you're going fast. For fast airplanes, wing sweep delays shock waves and airflow separation which do bad things.

All bets are off for any fighter from the F-16 on. They are built to be unstable because that makes them able to turn (change attitude) faster than a stable airframe. The pointy end only stays in front as long as the flight computers are running.

Technically, there's lots of rhos, Reynold's numbers, squares &etc. but for models:

If it balances a little bit ahead of a point that is 1/3 back from the wing's leading edge, it should fly just fine. Mid-wing flying models need dihedral to fly wings level.

Yogi (ex-cathedra)

[CharlieC]

I don't think anyone's commented yet that the fins on propeller driven aircraft are often offset from the centreline to partially account for the torque reaction. From memory the T-6 Texan (Harvard to colonials) would fly with nearly neutral rudder trim at cruise power settings because of this.

Regards,

Charlie

[Retired_for_now]

Good point CC,
The rudder can be angled to counteract engine torque at normal speeds - you can also do it with a fixed tab on the rudder set at a slight angle (you'll see them a lot on little experimental airplanes - never give them a tweak!).
Yogi

[Zathros]

Angle of incidence on main wings negates heavy pressure on elevator to keep plane flying level.. Properly trimmed you can fly most planes with 2 fingers. The Corsiar (Jet) had a variable incidence wing to help with carrier landings. A plane of which far too few were made. Why dogfight a Mig when you could take 1,000 mph 6 miles wide turn, go up to 20,000 feet and come back and fight a battle on your own terms that a Mig could not possible handle. Rutan's airplanes used a laminar flow wing and there was one at Danbury Airport, Ct, that had a stripe painted on by the owner along the leading edge and could not get off the ground because of the disruption to the airflow. He almost hit the fence at the end of the runway.

[Retired_for_now]

Good points Z - but you're thinking in terms of control pressures. An aerodynamically stable airplane (almost) always has an inherent nose down pitching moment due to the location of the center of lift on the wing - behind the center of gravity. You don't normally notice this on the controls because the horizontal stabilizer/elevator is either set or trimmed with a negative angle of incidence to create an inherent downforce from the tail (pitch up) that balances the main wing's tendency to pitch down at normal cruising speeds. If you wander the flight line, you can even find airplanes where the horizontal stabilizer's profile looks like a upside down normal wing so it naturally produces negative lift and pushes the tail down.

The balance changes as the speed changes, which is why you mentioned trimming the airplane to fly level - and why the F-8 Crusader wing jacked itself up for landing (more factors involved - but ...). The angle of incidence built into the main wing also has a lot to do with the angle you want the fuselage/cabin to be at when cruising - at that point the wing's angle of attack is constant to produce the needed lift (lot's more rhos, v squared, C sub L, and other stuff).

Yogi (preachin' to the choir)

[Don Boose]

Facinating information. This thread is a keeper for anyone with an interest in full-sized aviation and aviation history. Thanks to all who contributed and especially my Air Force comrade in arms for the very thorough and highly readable aeronautical engineering discussion.

Don