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Old Jun 26, 2012, 12:41 PM
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I am a definite fan of V tails. It was said though that beech changed to straight tail because of torsional loads in the fuselage I guess a tubular fuse would better manage the torsional loads. A V tail certainly has the cool factor.
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Old Jun 26, 2012, 01:00 PM
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A Cessna has a low c.g. and small amount of dihedral which should produce a Dutch roll tendency but the yaw inertia is very low due to the short, light wings, while yaw damping is very great with the big fin and large fuselage side area and lastly, the lateral center of area is not far above the c.g.

A jetliner is very unstable due to the sweep/dihedral you mention but sweep is needed for structure and mach effects while dihedral is needed for ground clearance. So the only solution is to use an autopilot. Jetliners can be flown manually in an emergency since the Dutch roll oscillation is slow enough for the pilot to counteract but it would be very dangerous and the cabin would fill with vomit within minutes.
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Old Jun 26, 2012, 01:39 PM
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http://www.boeing.com/companyoffices.../k63304-3.html

Lots of dihedral in this front view. On the ground where the ground clearance is needed there is a lot less dihedral.

Dihedral plus sweep, I guess being too stable becomes unstable?

Recent accidents have shown that u r right, pilots r losing control when the autopilots stop working. But for certification would they not have to prove they r safe without autopilots even at high altitudes?
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Old Jun 26, 2012, 01:56 PM
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Dihedral is needed for ground clearance when the plane is in the air, not on the ground. And yes, Dutch roll and spiral mode are both cases of excessive stability causing instability.
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Old Jun 26, 2012, 01:58 PM
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U RSI right!
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Old Jun 26, 2012, 01:59 PM
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Sorry auto complete...automation saturation...I meant to say u r so right
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Old Jun 26, 2012, 04:06 PM
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The vertical C/G location is a relatively weak player in this game. You're thinking of "pendulum stability", which is mostly a myth, unless the plane is extraordinarily flexible, to the point that aeroelastic behavior is a major player (such as on the Goodyear "Inflatoplane").

The main reason for differences in dihedral between high wings and low wings is aerodynamic interactions between the wing and the fuselage, which can act like dihedral for high wings and anhedral for low wings. This is why the Whitman "Tailwind" could be controlled in roll by rudder only, despite having a Hershey bar wing and zero dhedral.

Vespa, dutch roll is the result of too much dihedral effect (which can include sweep effects) and/or not enough vertical tail effect.

Not enough dihedral and/or too much vertical tail results in spiral instability.
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Old Jun 26, 2012, 04:47 PM
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Yes, of course. I was responding to the captain's remark about Cessnas having high wings, low c.g. and a small amount of dihedral, all of which produce Dutch roll tendencies. The key here is "a small amount of dihedral", not to be confused with "too small amount of dihedral".
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Old Jun 26, 2012, 06:16 PM
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The other thing you need to bear in mind is that for full-scale in particular, the wing weight is one of the dominant factors in the overall weight of the plane. The C/G of a high-wing Cessna is probably not all that low.
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Old Jun 26, 2012, 07:09 PM
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So true, hadn't factored in wing weight.

This caught my eye, almost no dihedral

http://www.hobbyking.com/hobbyking/s...ampaign=zephyr
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Old Jun 28, 2012, 11:24 AM
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OK. I hope to get a couple answers from such experts as yourselves. I have owned two airframes many years ago, and flew some others that had "V" tails. They were RTF's (sorry).

I only scratch build now. I mostly build and fly sport planes, warbirds, and such, 98% electric.

Now the specifics. I have done a mold and have made a pod for a pod and boom motor glider. I case you wonder 42" wingspan, about 265 sqin area, and the wing is shaped very similar to a MiMi DLG, poly-dihedral, and this will be 3 channel, R/E/T. I can design great flying sport planes, and understand proportions of CONVENTIONAL airframe lengths and areas.

So questions. To do a "V" tail, thinking at 110 degrees for angular spread, and make them large enough to include the combined area of a conventional "X" tai (add stab and rudder areas togetherl, what is a good aspect ratio? On a sport plane I try to go horizontal stab, AR of 3 1/2 to 4. I would guess a motor glider would be more around AR 4 for the 'V" tail.

Maybe it should be more? I would also guess that the tailerons (if that is correct termenology) should be larger than 25% of the tail cord. Maybe 30%.

Any help or suggestions would be much appreciated.

Fred
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Old Jun 28, 2012, 11:59 AM
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Those numbers are reasonable. Your wing aspect ratio is fairly low (6.657), which says 110 degrees included might be a little small, you might want to flatten it out just a little more.

Horizontal tail requirement is related to wing chord, while vertical tail is related to wing span. Thus, if you have a low aspect ratio (lots of chord, relatively low span) you need more horizontal tail effect and less vertical tail. For a V-tail, the tail dihedral controls how much tail effect is allocated to horizontal vs vertical, therefore a need for less vertical results in a flatter tail angle. On our "Nymph" Mosquito class HLG (which had a relatively low aspect ratio wing to try to keep the Reynolds numbers reasonable) the tail dihedral was only about 30 degrees per side, so about120 degrees included angle.

Aspect ratio of 3.5-4 for the tail panels sounds reasonable. Again, it's a tradeoff between span effects and Reynolds numbers. Also, keeping the tail span down reduces the torque on the tail boom during a rudder input. Also, remember that a V-tail does better than a conventional or a T-tail in the span loading department. A longer tail moment arm reduces the required tail area, making the tail panels smaller, which reduces their span and also makes tail Re's more of an issue, so even lower tail aspect ratio.

At model Re's, the hinge line for a control surface needs to be much further forward. That 20-25% of chord guideline comes from full-scale practice. For our applications, much wider control surfaces work better, at least 30%, 40-50% is certainly reasonable, and 70% is not outlandish from an aerodynamic standpoint. Structural considerations often become the limiting factor.
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Old Jun 28, 2012, 12:18 PM
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Don,

Thank you so very, very much.

I understand completely from your most excellent explination, and your ability to make it understandable is applauded.

Flattening the tail to 120 degrees included makes sense because my wing is a fairly low AR for a glider. The cord is very generous. I will go 120 degrees for sure. That explination is worth a pot of Gold.

I rarely use 25% for elevators, even though I know I wrote that (from my favorite referance drawings). I was thinking you might respond with 30%, but the 40-70% was a mild shock! Does make sense, and I do not question that either. 70% would be difficult for my sturcture as I intend to use foam (spar probably not, glass coverd, probably yes). I will lay out a AR of 4, with a calculated correct (hopefully) area, and see what 30%, 40%, or ugggg 50% look like. Then a TLAR will probably take over, and with a carbon boom, the tail could always be removed to try another.

My tail boom right now is rough cut so it would allow at least a 2.5 (+/-) times the average wing cord distance to place my tail from the TE of the wing. It isn't high tech because I am using an arrow shaft for my boom, but it is very stiff. I have no fear my boom, pod, and joints can withstand any flight loads, and probably landing loads I could throw at it!

I will finalize my boom length, AR of "V" tail, taileron width, and such later today when I get back to the bench. I am about 75% completed on my design build, if not more.

You are a gentleman, and a life saver. I probably would have gotten close, but this makes me feel like we can fine tune the design much faster and more accurate because of your help.

Fred
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Old Jun 28, 2012, 12:23 PM
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I have to add, my glider experiance is faily weak. My design of gliders is new territory for me too. But I have no fear.

Fred
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Old Jun 28, 2012, 12:39 PM
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As far as the chord of the control surfaces, it's a Reynolds number thing, as I indicated. At full-scale Re's, the flow is turbulent, so it's easy to keep it attached. For us, the flow wants to be laminar, which means it's very easy for it to separate. The flow is losing kinetic energy through skin friction as it flows aft along the surface, which makes it increasingly more prone to separation the further back it gets. A well-aft hinge line requires more angular control deflection to get a given amount of % increase in camber, and that greater angular deflection makes the flow more likely to separate.

Moving the hinge forward puts it in an area with more kinetic energy (and therefore better able to handle the disturbance of the hinge) and also reduces the amount of angular deflection needed for a given control input, which makes it easier for the flow to stay attached. Less drag, better control authority.
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