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Pilot-34

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A. Will simply extending the tail make for more stable ground handling in a tail dragger and more stable flying in everything?
B. Well extending the tail make for a larger useful center of gravity.
C. Will increasing the size of the surfaces do the same as A and B ?
D Would it be worthwhile to use streamlined tubing in the verticals and angular reinforcement of a open tubular fuselage ?
Feel free to point out any other significant tail design points.
 

challenger_II

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(A) Yes, especially if you need to offset the weight of a heavy engine
(B) Not really. CG range is more dependent upon Center of Lift, i.e: width of wing chord
(C) Yes, but at the expense of added weight, and drag
(D) Yes, if you can handle the additional weight of streamlined vs round tube

A. Will simply extending the tail make for more stable ground handling in a tail dragger and more stable flying in everything?
B. Well extending the tail make for a larger useful center of gravity.
C. Will increasing the size of the surfaces do the same as A and B ?
D Would it be worthwhile to use streamlined tubing in the verticals and angular reinforcement of a open tubular fuselage ?
Feel free to point out any other significant tail design points.
 

Pilot-34

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I see extending the tail to deal with weight and balance out of the acceptable center of gravity as kind of like the difference between a single big steel weight and weights at the end of barbells.
It seems like concentrating weight near the center of the lift would be preferable to balancing at the end long arms
 

wsimpso1

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A. Will simply extending the tail make for more stable ground handling in a tail dragger and more stable flying in everything?
B. Well extending the tail make for a larger useful center of gravity.
C. Will increasing the size of the surfaces do the same as A and B ?
D Would it be worthwhile to use streamlined tubing in the verticals and angular reinforcement of a open tubular fuselage ?
Feel free to point out any other significant tail design points.
You have gotten answers, but I will answer with the reasons:

A is extending the tail arm - the distance from CG to 1/4 chord of the tailplane you are talking about - which increases tail volume and moves the neutral point aft. If your CG does not move, shifting the neutral point increases your static margin, which increases in-flight stability. For taildragger ground handling, this move will both make the tail more effective and make a cross wind weathervaning worse;

B this will usually widen your useable CG range. The aft shifted neutral point allows CG to be further aft befor your static margin becomes too small, and the bigger tail volume - tail area times tail arm - allows you to stall at CG’s that are further forward too;

C the concept of tail volume is in play here. Tail area times tail arm is tail volume so area and arm have equal weights in the scheme. Normalize by then dividing by wing area and either MAC (for horizontal tail) or wing span (for vertical tail). Then you can compare any new design to existing airplanes. Paz has a collection of airplanes with this computation and his recommendations.

Going further with this, you can make enough tail volume with a short arm and big tailplane or a longer arm and smaller tail. Usually smaller tailplanes make for lighter airplanes and less wing lift. They also tend to have more damping in that axis, which goes by area times arm squared. If you keep extending the tail arm and making the tailplane smaller, at some point, it’s weight will stop decreasing and start increasing. Stop extending the tail at that point.

D usually, streamlined tubing in the tailplanes is a a losing proposition except In external bracing. Usually we are trying to get certain stiffnness in bending of the tail plane. Streamline tubing of any given wall and minor diameter is close to the bending stiffness of a round tube of same diameter and wall. So it usually just adds weight when used inside the airplane.

Billski
 

TFF

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The balance of the arms is always a necessity. That never changes. Centralized weight is always what is being done, but you still have to balance.

Simple Cessna. People and fuel sit close to the CG. Flying does not change a lot when these vary. Engine sits about a third forward and tail about two thirds back. Tail is back to aerodynamically balance the wing, and it’s back to balance out the engine weight.

Let’s put the engine in the middle. That’s a big chunk that would be nice in the center. People or things have to go somewhere. Do we want them front or back of the engine? If we go behind, what balance do we have to the CG? If you put fuel up there instead of wing tanks, when you run low, the plane goes unstable. You could just stick a chunk of lead out there. That would solve it, but now something is getting a free ride that is not contributing to anything but CG. Put people in front. Good view. One person only, and the CG will be aft. All people and it’s probably nose heavy. You have to come up with a safe placement to be flown in any configuration. It probably never flys good. The balance is never at its best unless you carry some ballast. Might be a necessary evil to get what you want out of a design. Doubtful if will be fondly remembered as a great plane.

Design requires things. You need wings, a place to sit, fuel, engine, and it all has to balance. Flying wing is as short a tail and engine overhang as it gets. Still works the same. The shorter it gets the more perfect CG has to be. CG range does not play games with stubby. Long tail is the same thing as a long pry bar. Lots of leverage and can overcome some bad CG decisions long enough to fix them compared to a wing only. Short has no leverage.
 

WonderousMountain

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Sooo... My mind has been on SnS - Guppy Tail lately.

Their earliest design was Wood, however, Empannage was chromoloy 4340 bent tube. What's wrong is no directional heading pedals Off, so you just sort of meander around in the wind. Suggested fix was a Constellation Tri-tail. All good,

My thinking is to make these outer fins symmetric, orthagonal and without incidence or trim control. In this way the idea is simplified. However, it won't do, airfoils that are made of round tube & flats troubles a civilized drafter.

However, now I am in complication territory, trying to decide between traditional foil sections, with the flap cut out at some chord local, or a section that closes out with maybe 3/4" height and running a long flap aft. Now you might think there is some report on this, but I haven't read many all that recently. Then the big question, will this really do anything, or is swatting the air the same everywhere in the world?
 

Lars Odeen

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In regards to tail length, if you were building a design like the pietenpol air camper wouldn't it make sense to shorten the tail a bit if you weren't using the heavy model A engine?
Or am I headed off into the weeds here? Just trying to get a better understanding of this stuff.
 

TFF

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The Pietenpol’s tail length is already set for the Ford engine. When people use something like a A-65 they lengthen the engine mount so the airplane will not be tail heavy. If you shorten the tail, the tail surfaces need to be bigger to compensate. Not really going in the right direction. You can also get too close. Even if calculations say it will work, tucked too close looses margins of error.
 

rv7charlie

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For taildragger ground handling, this move will both make the tail more effective and make a cross wind weathervaning worse;
Also, if gear track width doesn't change, longer tail can increase risk of ground loops. It gets easier to get outside the wheel track, and harder to recover due to inertia.
 

Lars Odeen

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The Pietenpol’s tail length is already set for the Ford engine. When people use something like a A-65 they lengthen the engine mount so the airplane will not be tail heavy. If you shorten the tail, the tail surfaces need to be bigger to compensate. Not really going in the right direction. You can also get too close. Even if calculations say it will work, tucked too close looses margins of error.
By too close you mean the engine mount length?
 

TFF

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You can get the tail too close trying to compensate for the lighter engine weight. There are sweet spots. Engine weight is just a weight problem. Tail size and length revolves around aerodynamics just as much as weight. The tail is “ picking up” the CG that is forward of the center of lift that is the aerodynamic balance point. They are only inches apart or less. CG is makes the nose go down because it’s safer to control. The tail is an aerodynamic pry bar lifting the nose up. Add in pitching moment and Shortening the tail has a lot more consideration than lengthening an engine mount.
 

Lars Odeen

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This is starting to make sense now. I can see that there's much more to it than just taking off or adding a foot to the tail to compensate. It makes sense that the engine mount would be the easier option.
 

wsimpso1

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Sum of forces in vertical direction is zero
Sum of pitching moments is zero
First derivative of pitching moments with respect to pitch angle is less than zero
Sum of yawing moments is zero
First derivative of yawing moments with respect to yaw angle is less than zero

Satisfy all of this, and the airplane will fly nose on and want to stay there even when disturbed. Do not satisfy all of this, and it will tumble. Welcome to design.
 

Pilot-34

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Also, if gear track width doesn't change, longer tail can increase risk of ground loops. It gets easier to get outside the wheel track, and harder to recover due to inertia.
Wait wait wait!
A longer tail increases the risk of a ground loop?

I thought that was one of the problems with the tri pacer taildragger derivatives in that since they are so short they are Squirrley to land !
Are you saying that if we lengthen the tail they will become even worse?
 

Topaz

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Sum of forces in vertical direction is zero
Sum of pitching moments is zero
First derivative of pitching moments with respect to pitch angle is less than zero
Sum of yawing moments is zero
First derivative of yawing moments with respect to yaw angle is less than zero

Satisfy all of this, and the airplane will fly nose on and want to stay there even when disturbed. Do not satisfy all of this, and it will tumble. Welcome to design.
The fun thing is that you don't even need a tail to accomplish those things, thus flying wings. ;) Getting a single flying surface (the wing) to do all those things is also why flying wings are so hard.
 

rv7charlie

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Wait wait wait!
A longer tail increases the risk of a ground loop?

I thought that was one of the problems with the tri pacer taildragger derivatives in that since they are so short they are Squirrley to land !
Are you saying that if we lengthen the tail they will become even worse?
My thoughts; I could be wrong.
Never flown a Pacer, so I did a little searching. Apparently, there are a couple of gear widths. Are you talking about the wide or narrow gear?

In no-wind conditions, it's certainly more stabilizing, but as Billski said earlier, the longer tail will 'weathervane' more pushing the nose into the wind. Once you slow to the point where the rudder isn't doing much, if the tail starts sideways, there's a lot more inertia to overcome with the brakes.

I see it as a ratio thing; with wide gear and short arm, it's harder to get the tail outside the gear, and the brakes have more authority to recover. Ex: landing a 'hot' airplane like a Swift is almost as easy as landing a trike (please don't tell anyone ;-) ). Don't know, but suspect the Pacer's rep is based on the narrow gear.
 

wsimpso1

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Tail volume (area times arm) matters to both stability and control.

Static margin is essential to stability.

Tail volume coefficient is written up in Paz' "Light Airplane Design". TOWS has everything you need to calculate neutral point and static margin (which is how far CG is ahead of neutral point).

As for ground loop tendency, taildraggers have it. Learn how to keep the tail behind the bird. Narrower gear makes it worse.

Billski
 
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