How Can V-Tail Design Improve Stability and Control in Aileron-Less UAVs?

[Freddy Diaz]

Hi Physics Community,

I'm working on the design of an aileron less and tail engine small UAV (Wing span 800mm) but I got stuck in Stability and Control. I recall from a teacher that a V-tail design is good for those configurations. However, I cannot find references to calculate/estimate neither its overall size or surface control size and I'm not sure if it could be calculated with volume coefficient method.

I'd greatly appreciate any help or guide about this topic.

-Freddy.

 

[Danger]

Hi, Freddy.
Even though I was a pilot back in the mid 70's, I don't really know much about this aspect of aeronautics. The old Beech Bonanza was a steel-headed workhorse with a V-tail. It probably had a longer lifespan than most others, but it sure was weird to look at. One thing that you should consider is that most military drones have the tail in an inverted V. I think that it's to make sure that the pusher prop doesn't hit the ground, since the tips extend below the swept circle of the prop. That's all that I've got; you'll have to await the arrival of others for more assistance.

 

[rcgldr]

Without ailerons, you'll need some form of dihedral in the main wing (most of this can be near the wing tips), to create a roll response to yaw (rudder) input. (Swept back wings have some dihedral effect, but probably not enough for good stability). Depending on the model, V-tails may involve less drag than conventional tails, and they do produce a small amount of roll torque when used for rudder effect, but probably not enough for stability, so you're stuck with dihedral and it probably doesn't matter much if you use a V-tail or conventional tail.

 

[Freddy Diaz]

Thanks! I really appreciate your help. I'll consider your answer an update the results. When I Ge There. :)

 

[Danger]

Oh, man... now this is driving me nuts...
I absolutely know that I read an article in "Private Pilot" or "Plane and Pilot"or similar magazine back when I was in the game based upon something about using the V-tail to trick the Beech into thinking that it was an elevon system so that it could be flown with frozen ailerons. Unfortunately, that is far enough back in my memory (and I was flying high-wing Cessnas at the time) that I can't remember anything about it.

 

[rcgldr]

I absolutely know that I read an article in "Private Pilot" or "Plane and Pilot"or similar magazine back when I was in the game based upon something about using the V-tail to trick the Beech into thinking that it was an elevon system so that it could be flown with frozen ailerons.

A Beechcraft Bonaza is/was a low wing model with a lot of dihedral (7 degrees) in the main wing, but in some articles I found on the internet, it apparently had low roll stability (slip recovery), leading to exceeding the maximum speed which would lead to a relatively high incidence of structural failures in the early models.

 

[Danger]

...leading to exceeding the maximum speed which would lead to a relatively high incidence of structural failures in the early models.

Wow! I was unaware of that. I just didn't like the things because of them looking .... incomplete. I always suspected that flying one would feel like riding a 3-legged horse or something...

 

[rcgldr]

Beechcraft Bonaza ... low roll stability (slip recovery).

I always suspected that flying one would feel like riding a 3-legged horse or something...

I don't this is an issue of V-tail versus conventional tail. The articles mention an overspeed condition but did not explain the root cause of the overspeed condition. I'm not sure how close the cruise speed is to the never exceed speed, but normally there's some margin here. Random disturbances can put an aircraft into a slip condition, where it's banked but almost going straight. The aircraft starts "slipping" to the side decreasing altitude and gaining speed. If the wings have dihedral, then the side wind pushes the upwind wing upwards and the downwind wing downwards, which should level the wings and eventually stop the slip. However, if the sideways center of lift of the fuselage is above the center of mass of the aircraft, the fuselage generates a torque that opposes the correctional response from the dihedral in the main wing. There's also a yaw response to a slip, as the weathervane effect on the vertical component of the tail should yaw the plane into the direction of the slip, putting the plane into a mild dive, and as the plane picks up speed, the elevator trim should pitch the plane upwards to reduce speed (called pitch stability, the elevator is trimmed for a given speed in level flight). In the overspeed case, the typical self correction to a slip doesn't occur, and the aircraft picks up speed, part of which is a sideways component. It may be that the structural failures were related to a combination of forwards and sideways component of relative air flow.

I fly radio control gliders, both V-tail and conventional (cross) tail, and the V-tail models aren't any more difficult to control than the standard tail models. For both models and full scale aircraft the response to pilot input controls would be adjusted for a reasonable amount of sensitivity. For models, the amount of sensitivity is adjustable, since it's a fly by antenna situation, where the stick to throw reponse can be electronically adjusted in the transmitter, and mechanically adjusted on the model (the servo to control surface linkage).

The amount of self stability (slip and pitch) is setup depending on how the aircraft is to be used. Aerobatic models have close to zero stability, so that the model will tend to fly in a straight line with centered control inputs regardless if it's flying level, downwards or upwards (if flying fast enough) and so that a gusting sidewind doesn't produce a roll response (due to dihedral). Dihedral increases drag, so gliders tend to have less roll stabiliy than powered aircraft (except aerobatic models).

Getting back to the original question, there are a lot of radio control aircraft that only use rudder and elevator, and rely on dihedral in the wings for roll control. You can see a variety of designs in the the models. Some have wings that smoothly curve upwards toward the tips, while others have a 3 section wing with a flat main central wing and flat wing tip sections that are angled upwards, or a 4 section wing with a small dihedral in the central halves of the wing and greater dihedral at the tips (these are called "bent wing" models).

 

[Danger]

I'm not sure how close the cruise speed is to the never exceed speed, but normally there's some margin here.

Boy, is there ever! And nobody ever knows just exactly what it is. (One of my friends flew in the Battle of Britain and swears up and down to this day that he broke Mach in a Spitfire during a power dive. It seems strange to me, but I'm in no position to call him a liar.)
I've never had a plane go into a slip that I didn't deliberately put it into, and didn't realize that such things happened. (By the same reasoning that almost any commercially available bird is inherently "spin-proof"; you have to put some elbow grease into making it happen.) I really only did that for cross-wind landings, rather than for fun, since I was almost always using a municipal commercial airport. (My signature refers to one of the rare exceptions. :D) Slipping did, however, account for one of the sweetest things that I've ever seen. Test pilot and company front-man Bob Hoover performing what he called "The Tennessee Waltz" in his Shrike. Just drifting down the runway as cool as a breeze, touching one main wheel, then the other, back and forth like a ballet dancer. I got him to autograph my wineskin, and even though it fell apart 30 years ago I kept that one piece of leather. (I don't collect autographs for the usual reasons; this was a sign of my respect for one of the most talented pilots that I've ever met.)
Also, I don't want anyone to think that I dislike Beechcraft. When I was planning to start a charter service, and even all of these decades later as an individual who can no longer fly but would love to have something to look at in the garage, my heart has been set upon an E58 Baron with the Excalibur package. After the Avro Arrow, the P-38 Lightning, and the F-5 Freedom Fighter, it's my favourite bird ever.

 

[RandomGuy88]

Boy, is there ever! And nobody ever knows just exactly what it is. (One of my friends flew in the Battle of Britain and swears up and down to this day that he broke Mach in a Spitfire during a power dive

This seems very unlikely. The drag rise as you approach M = 1 is so drastic that the additional thrust from gravity would not make up for it. Many WW2 aircraft could certainly reach very high subsonic mach numbers and there may be a few that did actually reach M >1 but I am not sure about that. I think the highest recorded Mach number for the spitfire was somewhere around 0.9. It is possible that the instruments on the aircraft indicated a higher airspeed then what was actually achieved due to the effects of compressibiltiy on the pitot probes. In addition to the drag rise there are other very severe problems in the transonic region including a changing aerodynamic center resulting in changing stability characteristics (typically in a detrimental way) the increased loads could also cause the wing tips to twist downward so much that the effects of ailerons would actually reverse (this is called aileron reversal).

Getting back to the original question, there are a lot of radio control aircraft that only use rudder and elevator, and rely on dihedral in the wings for roll control.

Dihedral is used for roll stability not roll control.

I'm working on the design of an aileron less and tail engine small UAV (Wing span 800mm) but I got stuck in Stability and Control. I recall from a teacher that a V-tail design is good for those configurations. However, I cannot find references to calculate/estimate neither its overall size or surface control size and I'm not sure if it could be calculated with volume coefficient method.

So you are trying to compensate for your lack of ailerons by using a v-tail? Why, I am just curious?
You would certainly lose a lot of roll control authority due to the much smaller moment arm. A problem with a V tail is that it is not as good for lateral stability/control as a horizontal stabilizer of the same size and it is not as good for directional stability/control as a vertical stabilizer for the same size. The v tail surfaces have to be larger in order to account for the fact that the force vector is tilted.

A good reference for stability and control (that also talks about v-tails) is Mechanics of Flight by Warren Phillips.

 

[rcgldr]

there are a lot of radio control aircraft that only use rudder and elevator, and rely on dihedral in the wings for roll control.

Dihedral is used for roll stability not roll control.

On a rudder / elevator radio control model (no ailerons), dihedral is used to allow rudder (yaw) inputs to result in a roll response (yaw to roll coupling). So it's the combination of rudder input combined with dihedral that allows roll control. Perhaps I should have worded that better in my earlier post.

 

[RandomGuy88]

On a rudder / elevator radio control model (no ailerons), dihedral is used to allow rudder (yaw) inputs to result in a roll response (yaw to roll coupling). So it's the combination of rudder input combined with dihedral that allows roll control. Perhaps I should have worded that better in my earlier post.

Ah, ok that definitely makes sense.

 

[Danger]

the instruments on the aircraft indicated a higher airspeed then what was actually achieved due to the effects of compressibiltiy on the pitot probes

That one never crossed my mind; good theory. I think that he honestly believes it, and with people shooting holes in you it's hard to distinguish something like a sonic shock from a .30 cal impact when you have more adrenaline than blood running through your veins, but it never seemed quite right to me. The only other thing that I could think of was maybe needle-bounce on the panel.