does inverted V-tail aid turning

[ciurpita]

i hope i'm correct in saying that when a v-tail is used to yaw an aircraft, the elevators moved in opposite directions create a torque that forces the aircraft to roll. but the roll force caused by the v-tail is opposite to the bank needed to turn the aircraft.

i think that an inverted v-tail would create a roll force in the same direction as the bank. is this true, and if so, how big a force might this be? and if it's not insignificant, would it require less yaw to create a turn on a polyhedral?

[macboffin]

The primary force result with vee tail "ruddervaters" is in the yaw plane ; due to the short span of the tailplane pieces relative to the wing span, the roll (Torque) input is very small ; a downwards vee does assist the roll in theory, but in practise, flying the same model with the Vee tail mounted either way up shows very little difference. (Upwards) vee tails reduce the possibility of ground strike / damage on planes with no undercarriage,(ie gliders) and since two surfaces rather than than three, turbulence due to the intersection with the rear fuselage is less, so overall drag is less. Vee tails also lend them selves to mounting small/light cameras such as the Flycam in the fin thickness, especially where a swept forward wing ensures a clean field of view for the camera. As below.

[MCarlton]

Remember the "roll" comes from the instigation of "yaw" on a RE model, so you're not looking to create a roll effect and any such effect would be barely noticeable on a long polyhedral wing.

Likewise a conventional rudder would create a similar "torque" effect but this is not noticeable in most cases, although its worth pointing out that the effect is enough to warrant those in pursuit of aerodynamic purity trying to have as much rudder below the fuselage datum line as above it.

The other thing to note is that anhedral, which is what an inverted "Vee" would be, responds in the opposite way to dihedral in terms of the yaw induced roll, so that would suggest a roll couple in the opposite direction to the turn, so I think all in all everything would come out roughly balanced.

It is also possible that an inverted "V" tail would have a destabilising effect, as the tailplane would want to invert itself back to conventional dihedral, so there might be reduction in lateral stability also.

[Brandano]

I remember seeing an RC twin boom rudder and elevator only glider, that had two inward canted tails on the end of the booms not joined at the tips but left free. This allowed the booms to be spaced enough that the roll on rudder input was actually rather large. On top of that the booms, not being bound to each other, would tend to twist the wing and add even more roll.

[MCarlton]

I wonder at what point, or rather, what angle, does a v-tail stop providing a rudder function and start providing a taileron effect (ignoring the lack of "fin" area as angle increases) most seem to be between 100 and 110 degrees, but I saw an experimental model once with an almost flat "Vee" tail and I wonder how it went. I can't remember who did it, it was in Silent Flight mag ages ago and was on a lightweight balsa HLG, it was one of the regular columnists anyway

[Texas Buzzard]

This thread reminds me of an experience I had about 8 months ago.

I got hot for a 30" w.s. foamie (electric) for the F-22. I had no Ailerons.

I was using the two "ruddervators" for control. I had scaled up a set of 24" plans to 30" w.s. The day came for testing. There was about an 8 mph wind blowing.

The launch was good and I was climbing to 3 mistake high altitude. The turn out of the wind at full power was OK. But I reduced the power as I went downwind some.

Then we realized that this bird was going downwind with no ability to turn right or left. With full left rudder it wouldn't turn. I forgot to add power.
I got her down about a quarter of a mile away from launch by chopping power completely.
The F-22 just would not turn. No yaw, no bank - no turn. Upon looking things over I saved the model by adding humongous rudders and putting in more throw.

The moral of this story is that centrally placed "V-Tails" or "Ruddrvators" just don't have the torque needed to FORCE a bank. Others have said this.

You will not get any more responsiveness from The Inverted V-tail than the upright V-Tail. The obvious "fix" is to use ailerons - they have the torque.

[JetPlaneFlyer]

The moral of this story is that centrally placed "V-Tails" or "Ruddrvators" just don't have the torque needed to FORCE a bank. Others have said this.

No rudder has the 'torque' to force a bank... that's not how a rudder works. Any rudder works primarily by creating yaw. In an aircraft with ample axial stability (dihedral effect) this yaw then leads to a rolling torque generated by the dihedral which banks the aircraft. The slight rolling torque on a conventional rudder is in fact in the opposite direction to the desired bank.

This is why aircraft that have no dihedral (probably like your F22) dont turn well using rudder only. Generally aircraft with no dihedral only yaw when rudder is applied but they dont bank and show little inclination to turn.... Which is why they are invariably fitted with ailerons (or tailerons).

[Brandano]

On an F22 the V formed by the rudders is relatively narrow, and in any case it's a standard V and not an inverted V. Any roll coupled with yaw would be opposite to the yaw itself. Ok, maybe opposite isn't the correct term, as how can roll be opposed to yaw? It would be opposite to the roll input normally given for a coordinate turn. A widely spaced inverted V tail with an angle bigger than 90 degrees between the tails probably would add in some roll coupling. YMMV

[trooper8]

I'm thinking of using the ^ elevators of the F4 phantom in the inverted "V" tail control theory and wondered about the yaw VS roll effects. Was planning no ailerons for a profile foam parkjet. Ideas on how this would work?

[BMatthews]

Technically a conventional rudder has the same rolling action as the V tail you're looking at. At least it does on designs where the rudder area is located above the majority of the rest of the model. But as mentioned the moment arm it is working with is so short compared to the wings that there's no noticable effect. On most designs, especially any with dihedral, the effect is totally buried. On a few over modeling history the rudder rolling couple HAS been an issue. The old Top Flite Contender was known for having a rudder induced roll when the rudder was used with larger throw angles. This showed up especially in knife edge flight but was also documented in many magazine mentions during normal flight as well. Now there was likely more to it than JUST the rudder rolling component but it would have been a contributor to this.

As to how far out taileron surfaces would need to be to act like ailerons it depends on the wing span and how the lifting area of the airplane is distributed. If the wing is short and stubby then tailerons will have a very noticable effect. For example the full sized F111 uses tailerons during swept back flight instead of the ailerons. And fighters such as the F18 and F14 use taileron action as much for lowering drag as they do to enhance the roll response. Remember that scene from the rear in Top Gun? The one where the climbing F14 suddenly snaps a roll? ALL of the surfaces went hard over in that one. The ailerons, coupled flaps and the tail surfaces acting as tailerons.

However if the tail is less than around 1/2 a wingspan the effect isn't noticable. There was a guy in the Parkflyer forum here at RC groups a couple of years back that made a short and stubby flat foamie Pitt'ish sort of biplane. To simplify things he went with tailerons on a stabilizer that was about 60% of the wingspan... Yes it was HUGE! Results were that there was lots of elevator authourity but that the roll response was all but non-existent. It flew like a super lazy trainer and he managed to dork it a few times when it wouldn't roll out fast enough.

Why? Because when the tail tried to force the roll the wings would see a change in the angle of attack on each side that countered this torque input so they would damp the action.

Why ailerons work so well and why tail rolling doesn't is because ailerons do two things simultaniously that work together. The first is that the ailerons add camber at the tips to alter the lift of the wing in a pro-rolling manner. Second is that deflecting the ailerons alters the local angle of attack of that portion of the wing in a pro-roll manner. In effect you're using the ailerons to strongly warp the wing into a pro-roll twisted shape. However trying to roll the plane from the tail doesn't do this and the wing remaining straight will fight the action.

[Aphorism's Dream]

Please let us not use jargon phrases like "pro-rolling". If we mean to say that the resulting roll force is into the desired (control input [command]) direction it is simply understood to be a positive reaction force.



Technically a conventional rudder has the same rolling action as the V tail you're looking at. At least it does on designs where the rudder area is located above the majority of the rest of the model. But as mentioned the moment arm it is working with is so short compared to the wings that there's no noticable effect. On most designs, especially any with dihedral, the effect is totally buried. On a few over modeling history the rudder rolling couple HAS been an issue. The old Top Flite Contender was known for having a rudder induced roll when the rudder was used with larger throw angles. This showed up especially in knife edge flight but was also documented in many magazine mentions during normal flight as well. Now there was likely more to it than JUST the rudder rolling component but it would have been a contributor to this.

As to how far out taileron surfaces would need to be to act like ailerons it depends on the wing span and how the lifting area of the airplane is distributed. If the wing is short and stubby then tailerons will have a very noticable effect. For example the full sized F111 uses tailerons during swept back flight instead of the ailerons. And fighters such as the F18 and F14 use taileron action as much for lowering drag as they do to enhance the roll response. Remember that scene from the rear in Top Gun? The one where the climbing F14 suddenly snaps a roll? ALL of the surfaces went hard over in that one. The ailerons, coupled flaps and the tail surfaces acting as tailerons.

However if the tail is less than around 1/2 a wingspan the effect isn't noticable. There was a guy in the Parkflyer forum here at RC groups a couple of years back that made a short and stubby flat foamie Pitt'ish sort of biplane. To simplify things he went with tailerons on a stabilizer that was about 60% of the wingspan... Yes it was HUGE! Results were that there was lots of elevator authourity but that the roll response was all but non-existent. It flew like a super lazy trainer and he managed to dork it a few times when it wouldn't roll out fast enough.

Why? Because when the tail tried to force the roll the wings would see a change in the angle of attack on each side that countered this torque input so they would damp the action.

Why ailerons work so well and why tail rolling doesn't is because ailerons do two things simultaniously that work together. The first is that the ailerons add camber at the tips to alter the lift of the wing in a pro-rolling manner. Second is that deflecting the ailerons alters the local angle of attack of that portion of the wing in a pro-roll manner. In effect you're using the ailerons to strongly warp the wing into a pro-roll twisted shape. However trying to roll the plane from the tail doesn't do this and the wing remaining straight will fight the action.

[coosbaylumber]

Well, more than a year ago, got to talk to Dave Brown about the Tiporare and the Curare' both of which have drooping hoz-stabs. The idea there was to put a L.E. out into un-molested air, but had a secondary benefit too. It made for better knife edge flying they found. Per Dave, it was very Iffy, as to exactly what angle off the fuselage was taken and he had to move it one degree at a time to get to where they thought best. All changes involved tied together elevators on to one servo, all this before fancy radios permitted twin servos back there.

May be on old concept, but needs more digging into now.


Wm.

[Gannet AME]

In David Thurston's Book Design for Flying Second Edition he made the following statement based on the Mini Imp's Inverted V tail which is located below the fuselage in that the center of pressure on the Inverted V tail area is located approximately at the vertical center of gravity of the airplane tends to roll an inverted V-tail airplane into a banked turn. This means any tendency to be lax on rudder action is somewhat self-correcting on aircraft having inverted V tails.

[markdrela]

the center of pressure on the Inverted V tail area is located approximately at the vertical center of gravity of the airplane That's not right. The attached PDF how you can graphically determine the center and magnitude of the net effective rudder force of a Vtail. For an inverted Vtail, this center is well below the tail.

[zitron]

That's not right. The attached PDF how you can graphically determine the center and magnitude of the net effective rudder force of a Vtail. For an inverted Vtail, this center is well below the tail.

I was going to say that's a very good diagram, then I saw it's oh my... Prof. Drela! I have nothing further to add... :D

-Z-