Questions Regarding V-Tail Designs

[Matt308]

Traditional T-tail airplane designs are fairly straightforward for the novice to understand the physics of flight associated with vertical and horizontal stabilitors. Changes in these surfaces related to airflow produce a force moment resulting changes to airplane yaw and angle of attack.

But how is this accomplished with a V-tail? Each surface of the V-tail impinging upon relative airflow will result in a vertical and lateral force moment. The relative sums of these moments do not seem to intuitively result in a pure force moment affecting only yaw or only AOA. Must not be a terrible design since V-tails were used by Heinkel, Beech and recently in F-117 and other UAV designs.

So my questions.

1) Does anyone know where the physics of V-tail designs might be discussed in simple terms?

2) Does anyone have any information on mechanical rigging designs/schematics to give use an idea of the relative complexity of a V-tail versus a T-tail architecture?

3) For flight control systems, does this imply added complexity necessary to counteract these coupled moments (e.g., moments must either be counteracted manually/mechanically via ailerons movement)?

4) Do V-tail aircraft exhibit flight characteristics that are different from T-tail designs (only talking about mechanically rigged flight controls like Bonanza and not triple-redundant flight control computers use for inherently unstable designs like F-117)?

[Civettone]

I can only answer number 4 as I know a bit about the Bf 109 which was tested with a V-tail. The only benifit for the V-tail was that it was aerodynamically cleaner. It increased the top speed of the Bf 109 though only a few mph. There were some problems with the controls as they did not respond or have the efficiency of traditional tail surfaces, and that's why the Germans didn't believe it was worth the change.

Kris

[Matt308]

And that is exactly what I would have predicted. Less drag in clean configuration since only two surfaces , instead of three. And during maximum deflection of the ruddervator, I would think that this would only add to drag due to needing to counteract the unneeded force moment (vice pure input of yaw and pitch via rudder and elevator not resulting in unneeded force moment).

[FLYBOYJ]

I can answer this in one word...

YAW

[Matt308]

On c'mon FBJ. I was relying upon you to write my Masters Thesis for me.

[FLYBOYJ]

I just did!

[Matt308]

I'm afraid that would only get me a 'B'.

[FLYBOYJ]

Hey you'll still pass!

[Matt308]

But I strive for excellence. I need another word!

[timshatz]

Matt, look around on the internet for stories about the V-tail Bonanza. Had a reputation for coming off in flight. Finally, somebody brought one back with half of it gone and they grounded the whole fleet. I think it was in the mid 90s. I think, but am not sure, that the failure rate was something like one in every 25.

Should be plenty of info out there as well as a couple of ADs.

[GaryMcL]

For what it's worth and partially working from a somtimes faulty memory -

Re #2 - The Bonanza has a mechanical mixer called a differential control assembly in the tail that blends pitch and yaw inputs to the surfaces. It'd be similar to a mixer used to blend roll and pitch in a delta wing's elevons. In that aspect it's a more complicated control system relative to independent cable or pushrod runs to a conventional tail. With a T-tail or cruciform tail you could handle linkages with bellcranks or cables and not have to deal with mixing inputs since you'd have conventional control surfaces. All you are doing there is changing the direction of the cable/pushrod runs. The downside to these tails is having to have a beefier vertical stab to handle the loads from the horizontal surfaces, especially a tall one like a Seminole as opposed to a stubby one like an F-104.

Re #3 - If the surfaces are moved together equally for pitch inputs, I think the lateral moments cancel each other out due to the angles of the surfaces being opposite. Right side raised yaws left, left side raised yaws right. Raised equally, right yaw moment cancels left yaw moment and yields only a pitch change. I would think there'd also be a large amount of drag as well due to the yawing moments fighting each other. Unbalanced inputs would result in yaw only or a yaw/pitch combination. The Bonanza actually moves both surfaces in opposite directions similar to ailerons for yaw (one surface up, one down). It's also a closed-loop cable system with no pushrods.

Re #4 - The early, shorter 4-seat Bonanza was/is known for a tail wag in turbulence that I think was attributed to the reduced vertical tail area being unable to damp it. I think the fuselage stretch in the later 6-seat V-tails at least partially corrected this.

The only Bonanza manual I have is a mid-60's S-35 V-tail. Unfortunately it doesn't have a control system diagram I can scan for you. The lubrication diagram for the differential control assembly isn't really clear enough to answer any questions.

Hope this is helpful. But it's probably worth just about what it cost.

I'll put my bunkers on now so I've got some protection from the flame.

Gary

[FLYBOYJ]

Good info Gary, I was going to chime in on this earlier. This was from the American Bonanza Society...

V-Tail Controversy
In June, 1984, the ABS requested the FAA to investigate the integrity of the V-Tail Bonanza. This was precipitated by several bonanza in-flight accidents that occurred within a short time span.
The project was undertaken by the FAA in the summer of 1984 and initially moved slowly. A decision was made by the FAA to hire Transportation Systems Center, Cambridge, Massachusetts, to study the Model 35. In 1985, TSC produced a written report which was largely a theoretical analysis of the aerodynamics and structure of the airplane and its relevant accident history. Beech opposed the publication of the TSC report because it contained errors, both in statements of fact and in analysis technique.
It was Beech’s opinion that the TSC report—aside from its errors—contributed little in the way of useable information to what was already known about the Model 35. In early 1986 the FAA and Beech agreed that actual testing of the Model 35 would be required if valid conclusions were to be drawn about the V-Tail Bonanza. Beech was asked to bear the entire cost of tests and to conduct the tests with the oversight and participation of the FAA.
The test resulted in the following recommended fixes:
1. Installation of a leading edge cuff for all C-Model aircraft and newer.
2. Installation of additional doubling material at the stabilizer/fuselage juncture for C through G-Models.
3. Resetting of trim tab stops to lessen the likelihood of a down trim condition that would exceed redline.
4. Special airworthiness examination of the V-Tail aft fuselage, empennage, ruddervators, and control cables to determine proper installation and operation condition.
5. Reweighing of certain aircraft as yet to be determined as part of the FAA Airworthiness Directive.
6. Additional baggage compartment and main entrance door placards.
The cost to Beech for the testing was approximately 1.8 million dollars and the V-Tail Bonanza became the most thoroughly tested general aviation aircraft ever built.

On 35 series there were also several ADs around the tail...

97-06-11 05/16/1997 Ruddervator Differential Tail Control Rod Assy
89-05-02 03/24/1989 Elevator Control Fittings

I believe the fittings involved replacing magnesium fittings with steel fittings.

[Matt308]

Originally Posted by GaryMcL

For what it's worth and partially working from a somtimes faulty memory -

Re #2 - The Bonanza has a mechanical mixer called a differential control assembly in the tail that blends pitch and yaw inputs to the surfaces. It'd be similar to a mixer used to blend roll and pitch in a delta wing's elevons. In that aspect it's a more complicated control system relative to independent cable or pushrod runs to a conventional tail. With a T-tail or cruciform tail you could handle linkages with bellcranks or cables and not have to deal with mixing inputs since you'd have conventional control surfaces. All you are doing there is changing the direction of the cable/pushrod runs. The downside to these tails is having to have a beefier vertical stab to handle the loads from the horizontal surfaces, especially a tall one like a Seminole as opposed to a stubby one like an F-104.

Re #3 - If the surfaces are moved together equally for pitch inputs, I think the lateral moments cancel each other out due to the angles of the surfaces being opposite. Right side raised yaws left, left side raised yaws right. Raised equally, right yaw moment cancels left yaw moment and yields only a pitch change. I would think there'd also be a large amount of drag as well due to the yawing moments fighting each other. Unbalanced inputs would result in yaw only or a yaw/pitch combination. The Bonanza actually moves both surfaces in opposite directions similar to ailerons for yaw (one surface up, one down). It's also a closed-loop cable system with no pushrods.

Re #4 - The early, shorter 4-seat Bonanza was/is known for a tail wag in turbulence that I think was attributed to the reduced vertical tail area being unable to damp it. I think the fuselage stretch in the later 6-seat V-tails at least partially corrected this.

The only Bonanza manual I have is a mid-60's S-35 V-tail. Unfortunately it doesn't have a control system diagram I can scan for you. The lubrication diagram for the differential control assembly isn't really clear enough to answer any questions.

Hope this is helpful. But it's probably worth just about what it cost.

I'll put my bunkers on now so I've got some protection from the flame.

Gary

Thanks Gary!! No flame from my end and the scan would be appreciated, but only if you find the time.

So do the mechanical mixers allow for independent ruddervator movement in the following combinations?

PR = Port Ruddervator; SR = Starboard Ruddervator

1) PR and SR neutral. No yaw or pitch moment.

2) PR "up"; SR neutral. Pitch up; yaw starboard.

3) PR "down"; SR neutral. Pitch down; yaw port.

4) PR neutral; SR "up". Pitch up; yaw port.

5) PR neutral; SR "down". Pitch down; yaw starboard.

6) PR "up"; SR "up". Pitch up; yaw moments cancel; increase drag.

7) PR "up"; SR "down". Pitch moments cancel; yaw starboard; increase drag.

PR "down"; SR "up". Pitch moments cancel; yaw port; increase drag.

9) PR "down"; SR "down". Pitch down; yaw cancel; increase drag.

So what sticks out to me is that the V-Tail appears to be somewhat inefficient and that during extreme maneuvering the ruddervators could be capable of oscillating from stop-to-stop depeding upon max yaw/pitch inputs.

Example scenario, upon take off you encounter windshear resulting in nose down pitch, you pull back the yoke (6) and apply power. Exiting windshear you encounter high winds at 090 and apply rudder (7). Thus the SR goes stop to stop.

Not being a mechanical engineer, the rigging and mixing of these movements must be fairly complex for mechanical systems rigged directly to the flight controls. This is where I would be most interested to see a schematic.

Thanks again, Gary. And I hope others weigh in and correct my assumptions if they are wrong.

[FLYBOYJ]

Check this out....

Borigging

[FLYBOYJ]

More info...