All flying "V" tail?

[Discharger]

I,ve searched the various forums for any information on this and nothing found. Well, I guess I'll find out soon enough how it works as I've almost completed a home brew e-sailplane with this type of tail. Rather than the common method of hinged control surfaces on each plane, I thought I would try pivoting each plane at the fuselage with the pivot point about 1/3 back from the LE. Any one have experience with all flying V's?

[Ollie]

I have built and flown planes with all moving V-tails. They work but they are draggier and have less control power than a V-tail with well designed, hinged control surfaces. This is because of air leaking between the tail and fuselage which effectively reduces the aspect ratio of the tail and produces turbulence.

If the pivot line runs through the aerodynamic center (25% of the mean aerodynamic chord) of the surface, the tail will be aerodynamically balanced and the servo load will be minimized. If the pivot line is aft of the aerodynamic center then the surface will blow back. This will increase the servo load and any dead band allowed by elasticity in the linkage. If the pivot line is located too far ahead of the aerodynamic center, the surface will flutter at some high speed.

[Discharger]

Ollie, thanks for your informative reply. The project is almost completed and I hope to have first flights in a couple of weeks time.

[Sail 'n Soar]

I have built and flown planes with all moving V-tails. They work but they are draggier and have less control power than a V-tail with well designed, hinged control surfaces. This is because of air leaking between the tail and fuselage which effectively reduces the aspect ratio of the tail and produces turbulence.



Ollie,

Are you sure about that? With moderately cambered foils and the aircraft balanced in the 25% to 30% AC range, the control surfaces are operating at a very low CL, so that the induced drag is very small anyway, minimizing the impact of reduced effective aspect ratio to near nil. In terms of turbulence, the question is, which is greater, the increased turbulence at the fuselage joint or the increased profile drag with the deflected control surface? The flying V-tail turbulence can be minimized by having them mate to flats on the fuselage over the normal control deflections, as many full scale airliners do.

You response already discussed what it takes to minimize flutter - in addition to balanced control surfaces, for large, fast models.

Other than my comments relative to the minimal impact of reduced effective aspect ratio, I don't really know the answers to the questions I posed. But my gut feel is that the full flying V-tail, if done correctly, would be the lower drag configuration of the two imost cases at model Reynolds numbers.

[Ollie]

Sail 'n Soar,

You are right. I over emphasized the disadvantages of all moving V-tails if properly done. The ones I did had concealed linkages or linkages in the wake of a small pylon in an attempt to minimize the drag of the linkages. As a result I ended up without the flats that would minimize the gap associated with deflected surfaces.

BTW, I like to set up with the CG nearer 40% of the MAC to keep the tail more lightly loaded at high speeds. My tail moment arms are about five times the average wing chord and my horizontal tail volume coefficient is about 0.4 to minimize the tail area. With this set up I only need a maximum of about plus or minus five or six degrees of articulated elevator deflection. So there is very little additional drag to slightly deflected, skin hinged elevators.

In any case I think we are talking about very small refinements in the context of the total drag that could go either way depending on the details of the particular design.

[California Condor]

Sail 'n Soar,. So there is very little additional drag to slightly deflected, skin hinged elevators.

In any case I think we are talking about very small refinements in the context of the total drag that could go either way depending on the details of the particular design.

Low drag and very good control. http://www.rcgroups.com/forums/newreply.php?do=newreply&p=2134137#
http://www.rcgroups.com/forums/newreply.php?do=newreply&p=2134137# ...Eut

[Sail 'n Soar]

In any case I think we are talking about very small refinements in the context of the total drag that could go either way depending on the details of the particular design.

Here, here :)

[banktoturn]

Ollie,

Are you sure about that? With moderately cambered foils and the aircraft balanced in the 25% to 30% AC range, the control surfaces are operating at a very low CL, so that the induced drag is very small anyway, minimizing the impact of reduced effective aspect ratio to near nil. In terms of turbulence, the question is, which is greater, the increased turbulence at the fuselage joint or the increased profile drag with the deflected control surface? The flying V-tail turbulence can be minimized by having them mate to flats on the fuselage over the normal control deflections, as many full scale airliners do.

You response already discussed what it takes to minimize flutter - in addition to balanced control surfaces, for large, fast models.

Other than my comments relative to the minimal impact of reduced effective aspect ratio, I don't really know the answers to the questions I posed. But my gut feel is that the full flying V-tail, if done correctly, would be the lower drag configuration of the two imost cases at model Reynolds numbers.

Sail 'n Soar,

You're right, induced drag is not the big factor here. I don't think that the all-moving configuration would end up being the lower drag though. The junction of the all-moving surfaces with the fuselage is difficult to seal, and will result in additional drag. Also, increasing camber is a more efficient way to increase lift than increasing the AoA. In principle, one could design the tail surfaces so that when the 'ruddervators' are deflected, the surfaces continue to operate in the drag bucket. I think this would be a better expenditure of effort than making all-moving surfaces work. The fundamental advantage of all-moving surfaces is that they allow the aircraft to maintain control authority at extreme AoA, or with extreme downwash on the tail surfaces. A secondary advantage is the ability to trim them to zero incidence when the tail surfaces need to generate zero lift. Since a well designed tail will be very close to that configuration with conventional control surfaces, the all-moving tail is likely to be higher drag overall.

banktoturn

[Sail 'n Soar]

Sail 'n Soar,

You're right, induced drag is not the big factor here. I don't think that the all-moving configuration would end up being the lower drag though. The junction of the all-moving surfaces with the fuselage is difficult to seal, and will result in additional drag. Also, increasing camber is a more efficient way to increase lift than increasing the AoA. In principle, one could design the tail surfaces so that when the 'ruddervators' are deflected, the surfaces continue to operate in the drag bucket. I think this would be a better expenditure of effort than making all-moving surfaces work. The fundamental advantage of all-moving surfaces is that they allow the aircraft to maintain control authority at extreme AoA, or with extreme downwash on the tail surfaces. A secondary advantage is the ability to trim them to zero incidence when the tail surfaces need to generate zero lift. Since a well designed tail will be very close to that configuration with conventional control surfaces, the all-moving tail is likely to be higher drag overall.

banktoturn

Good points, but one point you may not have thought of relates to increasing camber as the more efficient approach to increasing lift. That is true. As I see it, though, the question is what is the relative impact of increased drag while turning vs. while trimmed going something close to straight (what fraction of time is spent turning vs. going straight.) In terms of maintaining trim, in most cases the fold-cambered, aka, hinged flap control surface, is not the most efficient way to create the specific lift required from the tail. For example, if the CG is on the aero center, then the vertical component of the V-tale stab CL is practically constant. Any hinged flap deflection will, thus, create increased profile drag as the foil will then be operating at an AoA required to either reduce or increase the CL back to what it was prior to deflecting the contol surface. It's a little lengthier discussion, but the same basic thing occurs when the CG is within the normal 30% to 40% range.

For turning, with a rudder only yaw/roll control aircraft, then the deflected surface will be the less efficient way to generate the required yaw trimming CL than would the all flying tail. The one place where it is more efficient is for aileron equipped aircraft. In that case, the rudder function is to remove yaw. And in that case, the cambered foil would be the more efficient approach.

[banktoturn]

Good points, but one point you may not have thought of relates to increasing camber as the more efficient approach to increasing lift. That is true. As I see it, though, the question is what is the relative impact of increased drag while turning vs. while trimmed going something close to straight (what fraction of time is spent turning vs. going straight.) In terms of maintaining trim, in most cases the fold-cambered, aka, hinged flap control surface, is not the most efficient way to create the specific lift required from the tail. For example, if the CG is on the aero center, then the vertical component of the V-tale stab CL is practically constant. Any hinged flap deflection will, thus, create increased profile drag as the foil will then be operating at an AoA required to either reduce or increase the CL back to what it was prior to deflecting the contol surface. It's a little lengthier discussion, but the same basic thing occurs when the CG is within the normal 30% to 40% range.

For turning, with a rudder only yaw/roll control aircraft, then the deflected surface will be the less efficient way to generate the required yaw trimming CL than would the all flying tail. The one place where it is more efficient is for aileron equipped aircraft. In that case, the rudder function is to remove yaw. And in that case, the cambered foil would be the more efficient approach.

I don't think I quite followed all that. I don't see why the profile drag of a stabilizer with a deflected control surface should be assumed to be greater than the profile drag of a stabilizer at an AoA which gives the same CL.

banktoturn

[Sail 'n Soar]

I don't think I quite followed all that. I don't see why the profile drag of a stabilizer with a deflected control surface should be assumed to be greater than the profile drag of a stabilizer at an AoA which gives the same CL.

banktoturn

Here's a try at explaining. Things would be more obvious with a symmetrical winged aerobatic or racing AC, but since the original post was relative to a V-tailed sailplane, I'll give a related example. For arguments sake I'm assuming an efficient operating CL range between.3 at high speed cruise and .9 for minimum sink and efficient thermalling, consistent with the attached drag polar for E193 at Re = 200,000 That places the high speed cruise about 73% higher than the min sink speed. It also represents about a 5 degree angle of attack/trim difference. For sake of discussion I'm assuming the CG is at the 25% AC. (The math gets a little more involved if you assume something else.) For this notional sailplane design balanced at the quarter cord, assume that the stab CL = -.1 at Wing CL = .3 (the actual CL value will vary with the actual design specifics, but it really doesn't matter what the value is assuming you have designed your stab around that value.) To now fly trimmed at wing CL = .9, the aircraft will be at a 5 degree increased angle of attack, you will have to have given some up elevator greater than 5 degrees. But the stab CL will still be -.1, since for this example the CG and AC are aligned. That translates to the elevator deflected stab opperating at a negative angle of attack to get back down to the -.1 CL value without deflection, and that genereally results in a CD increase. In fact, that can easily double the CD at the trimmed condition for even a 10 degree deflection. (Note that the induced drag will be relatively constant because CL has not changed.) You can run the numbers at some othr balance point, but the results won't change that much. You still end up with the deflected tail surface trimmed model will have the tail operating at a less optimum AoA than it was before it was deflected.

Polar data from published Selig wind tunnel measurements