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Old Nov 07, 2012, 08:21 AM
Red Merle ALES
Curtis Suter's Avatar
United States, Mt, Helena
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Horizontal Tails

Thinking of horizontal tails in Thermal Duration models.

In the 1990's the horizontal tails were mounted in the fin and most were full flying. When the Supra came out with a full flying horizontal tail mounted in front of the vertical fin that became the norm for a few years. Now I'm seeing horizontal tails moving back to being mounted in the vertical fin.
Is this just builders preference?
What are the pros/cons of either method?
Is it a structural or aerodynamic consideration?

Thanks
Curtis Suter
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Old Nov 07, 2012, 09:30 AM
Flying = Falling (Slowly)
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The direct ancestry of the V-mount goes back at least to Drela's record setting Daedalus. I suspect that it was selected as the optimum way to minimize weight and maximize rigidity for mounting the horizontal surface. Also, as originally employed on the Bubble Dancer, control rods were mounted externally to eliminate penetrations of the end of the thin carbon boom. The original booms for Bubble Dancers had limited torsional properties and Drela did everything he could to not compromise strength -- even at the extremes of the plane.

I suspect that one of the reasons more planes are going to the fin mounted horizontal surface is because we have learned how to fabricate composite materials to incorporate the rigidity of the V-mount, maintain the structural integrity of the back end of the plane while still keeping the weight down. As revolutionary as the Bubble Dancer and Supra are, they were conceived at a time when much less was known about how far you could push the strength/weight properties of composite construction.

Happy Landings,

Don

BTW: It would require someone with a higher pay grade at aerodynamics than me, but I would also observe that Drela was always concerned about interference between elements of the plane -- hence the pylons for his designs. It is possible that the v-mount also was intended to reduce this kind of induced drag.
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Old Nov 07, 2012, 09:57 AM
Red Merle ALES
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Thanks Don.
It seems to me, also uneducated in aerodynamics that their would be more intersection drag with the horizontal mounted in the fin. With the horizontal on a V-mount ahead of the vertical tail then the down wash from the horizontal tail may make the rudder more effective thus able to reduce the size of the vertical. Perhaps the effects not noticeable with my hands on the sticks.

Interesting.
Thanks
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Old Nov 07, 2012, 08:19 PM
Turn down for what?
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Originally Posted by CloudyIFR View Post
Thanks Don.
It seems to me, also uneducated in aerodynamics that their would be more intersection drag with the horizontal mounted in the fin.
That's correct.

Full flying horizontal in the fin is easy to get done really well. It is easy to end up with a very stiff bell rank horizontal in the fin setup.

It is a little harder to get that right on a v mount setup.

Ryan
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Old Nov 07, 2012, 09:15 PM
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The tail on Daedalus is actually predated by the previous MIT human powered plane, MONARCH designed and built while Drela was still an undergraduate in 1983.
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Old Nov 07, 2012, 09:37 PM
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I wonder if anyone has done the trade off on the aerodynamic interference effects of having the stab on the fin (a negative mostly likely) offset by the additional tail volume coefficient one gets by having the stab further back (assuming same size) which would be a positive. I suspect somewhere there is something written by Dr Drela that would be beneficial.

I have only flown stab on the fin, never in front so have zero experience in the "stick" difference. I have seen the stab mounted in front of the fin - it looks quite fragile (from a durability perspective - whacking it getting in and out of car or house as I do).

Scott
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Old Nov 07, 2012, 10:15 PM
Flying = Falling (Slowly)
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I've had a dozen or so planes with V-mounts, half of which I built from scratch myself. In addition, I used to make and sell completely finished CF Bubble Dancer fuselages with V-mounts installed. They were all easy to install (once the v-mounts were molded). Alignment was a snap. And the only failure was an inexpensive mount that I badly abused.

In years past I have built a dozen or so planes including Aquila's, Sailaires and Cumics that have fin mounted stabs. I cannot honestly say that any of them were especially difficult to fabricate.

On the other hand, my latest plane - a Maxa - has a fin mounted stab and I doubt that very many of us could duplicate its combination of precision, light weight and strength without the CNC equipment which was employed to fabricate the original molds. A few dollars, though, and anyone can have one, no muss, no fuss

Happy Landings,

Don
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Old Nov 08, 2012, 01:49 AM
Turn down for what?
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Quote:
Originally Posted by ScottSails View Post
I wonder if anyone has done the trade off on the aerodynamic interference effects of having the stab on the fin (a negative mostly likely) offset by the additional tail volume coefficient one gets by having the stab further back (assuming same size) which would be a positive.
I am told by people that know things that the problem is a lot of aero stuff they don't have any way of knowing how good or bad something really is. But what they can know is if you eliminate it completely then you know it is good. So for example, lets think about the intersection between the wing and the fuselage. It might well be that if you have a nice blended intersection between the wing and fuselage like a fighter plane the air goes around that real well and isn't draggy. But you also might do that totally wrong and make things worse. The smart aero guys don't necessarily have a way to even measure or test this easily although perhaps the computer modelling is getting better these days. Or it well could be inaccurate too.

So instead what if you could have a magic way that using invisible force field tied the fuselage just below the wing and they were never connected. Now there is no wing/fuse joint at all. So that's real good from a drag perspective. The wing on a pylon is the closest to that magic invisible force field connecting the wing to the fuselage.

So the tail on a V mount is just another version of a magic force field that connects the tail to the fuselage. It is the closest that a person could get to that. How much worse or maybe not even worse at all the tail mounted to the vert stab is who knows. Maybe nobody does.


Ryan
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Old Nov 08, 2012, 07:03 AM
Herk
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Probably the best qualitative indicator is full scale practice.

A lot of fine tuning and development work is done on high performance competition sailplanes. Apart from the effects of low RN on models, what's best for manned saiplanes in terms of stability, control, drag and weight is probably best for a competition model sailplane as well.

However that all depends on how the model will be used. Full scale sailplanes do not experience DLG flight maneuvers. So, what's good for an ALES model or a winch launched glider is not necessarily what's best for a DLG model or perhaps an acrobatic sloper.
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Old Nov 08, 2012, 09:56 PM
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Quote:
Originally Posted by ScottSails View Post
I wonder if anyone has done the trade off on the aerodynamic interference effects of having the stab on the fin (a negative mostly likely) offset by the additional tail volume coefficient one gets by having the stab further back (assuming same size) which would be a positive. I suspect somewhere there is something written by Dr Drela that would be beneficial.

I have only flown stab on the fin, never in front so have zero experience in the "stick" difference. I have seen the stab mounted in front of the fin - it looks quite fragile (from a durability perspective - whacking it getting in and out of car or house as I do).

Scott
Static stability is proportional to the stab moment. (i.e. how far back the stab is). Damping is proportional to the square of the stab moment. Most sailplanes have a MUCH larger moment of inertia in yaw than they do in pitch, so they need more damping in yaw than in pitch. I suspect this is why the fin is put furthest back.

I think if in each case the design is done right, you wouldn' t notice the difference in handling. A lot of times, though, it isn't. Lots of too small v tails out there, and presumably errors in other configurations too.
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Old Nov 09, 2012, 08:42 PM
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Quote:
Originally Posted by lincoln View Post
Static stability is proportional to the stab moment. (i.e. how far back the stab is). Damping is proportional to the square of the stab moment. Most sailplanes have a MUCH larger moment of inertia in yaw than they do in pitch, so they need more damping in yaw than in pitch. I suspect this is why the fin is put furthest back.

I think if in each case the design is done right, you wouldn' t notice the difference in handling. A lot of times, though, it isn't. Lots of too small v tails out there, and presumably errors in other configurations too.
Thanks and interesting... I guess I haven't found a plane that has TOO large of a tail yet, too small - yes. I classify the Sagitta 900 in that category.

All the planes I have built I went overboard on stab area - I like stability and lots of damping
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Old Nov 09, 2012, 09:27 PM
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I had a 3 meter glider which I guess had heavy wing tips. Wallowed around and was difficult to fly slowly. A rate gyro on the rudder got rid of most of the problem. Easier than stretching the rear fuselage.
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Old Nov 09, 2012, 10:20 PM
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My Sagitta 900 wallowed around at slower speeds- I removed weight (extended nose and used a smaller battery) and put in a bigger stab - it helped tremendously.

I finally just built a 115" version of the Sagitta with MH32 with long tail.

Scott
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Old Nov 09, 2012, 10:27 PM
Red Merle ALES
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Thanks folks.

I take it there is most likely a neglible difference if the stab is mounted ahead of the fin or in the fin. Builders preference I take it.

Since some folks are talking about the size of your tails. There are some simple calculations that can be made to find Vv and Vh.
Visit the "Article/Files" page at www.TailwindGliders.com and download the article and file for Sailplane Calc.

Curtis
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Old Dec 03, 2012, 11:42 PM
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Latest Ships have fixed stabs with articulated elevators

Same as old school ships. Pike Precision and a bunch of ships are using fixed stabs with articulated elevator, same as the Genital Lady.

Full circle, but for good reasons.

CG mis information was a big one, it wasn't about CG it was about balance.

Stab Airfoils made a difference, aero elasticity a big factor.

Weight and simplicity drove the designs too.

The new ships don't need full flying stabs, so they can be simplier, lighter and therefor more durable and cheaper too.

Gordy
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