Control authority of V-tail vs. Cruciform - Page 3 - RC Groups
 Jun 21, 2012, 04:23 PM REMOVE TRUMP NOW! I don't know if you mean horizontal or vertical "tail", but neither is correct. If vertical, sweep increases the required size. If horizontal, sweep has no direct relation. Sweep adds pitch damping, but still it's the airfoil that must provide the pitch stability thru design or twist. And you can never have "sweep plus dihedral". Well, not more than once anyway.
 Jun 21, 2012, 05:19 PM Registered User no tail just what u see. elevons for control. just enough dihedral so that with 4 degrees pitch up the LE from root to tip remains same height (level with) above the sea.
Jun 22, 2012, 02:12 AM
Registered User
I love V-tails. I hope you guys dont mind me adding some pics of mine. My design method was to take a horizontal and make it bigger. I use 110 degrees for the angle. It isnt scientific, but has worked for me.

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 Jun 22, 2012, 07:21 AM Registered User A V-tail should have the same TOTAL area as the equivalent conventional tail. If you are taking the horizontal tail and "making it bigger" by an amount equal to the area of the vertical tail, then you're fine. If not, then your tail is too small.
Jun 22, 2012, 08:54 AM
Registered User
Quote:
 Originally Posted by Don Stackhouse A V-tail should have the same TOTAL area as the equivalent conventional tail. If you are taking the horizontal tail and "making it bigger" by an amount equal to the area of the vertical tail, then you're fine. If not, then your tail is too small.
I appreciate that is the proper way to do it. I used some numbers to help design my first one and have been using TLAR since. I havent been disappointed with stability or handling in the air, which is the goal.
 Jun 22, 2012, 09:43 AM Registered User As far as sweep and dihderal, geometrically you can have any combination of the two. Whether or not there is any advantage to doing so will depend on the details of each individual design. Aft sweep by itself does in most cases add static yaw stability as long as profile drag is dominant, as is demonstrated by typical swept flying wings with no vertical surfaces. However, if induced drag is dominant, then aft sweep can reduce static yaw stability. Aft sweep creates a dihedral effect (and forward sweep creates an anhedral effect). There is an old rule of thumb that three degrees of sweep is equal to one degree of dihedral, but this is incorrect. The dihedral effect of sweep is related to the lift coefficient, reaching a maximum at max lift, and zero at zero lift. Thus, an airplane using sweep for dihedral and for yaw stability has lots of dihedral effect and reduced yaw stability at high lift (a perfect recipe for dutch roll), and reduced dihedral effect and increased yaw stability at low lift/high speed (a perfect recipe for spiral instability). Adding more dihedral on top of the dihedral effect of sweep will worsen the low speed dutch roll tendencies. Aft-swept designs often have some anhedral to counteract excessive dihedral effect due to the sweep. Adding winglets also adds dihedral effect, requiring additional anhedral to balance that. Captarmour, if you dip an inward-canted winglet into the water during an episode of dutch roll, it's likely to dig in, precipitating a violent crash. Aft sweep does provide some pitch damping. Any time you have some flying surface forward, and some other flying surface aft, there will be changes in lift int he two regions proportional to the pitch rate, which will try to oppose the pitch rate, providing damping. This is true whether the longitudinal spread of the areas is due to chord, or sweep, or physical separation into two separate flying surfaces. The traditional way to get static pitch stability in a tailless configuration is by reflex in the airfoil trailing edge (so the trailing edge region acts like a horizontal tail), or by sweep plus twist (so the wing tips act like a horizontal tail), or some combination of the two. However, there are other ways, and I don't mean by resorting to artificial electronic stabilization.. Last edited by Don Stackhouse; Jun 22, 2012 at 09:50 AM.
 Jun 22, 2012, 03:25 PM Registered User ...I'm all ears...so far my first try will b just enough dihedral to keep tips out of the water at TO off pitch no vert tail, elevon hinge line swept aft to close 45 degrees, diff aileron to keep off the water in turns and to feed in some rudder effect. Remember too engine will b very high mounted which will lesson dihedral effect...
 Jun 22, 2012, 05:50 PM Registered User Differential aileron on a tailless aircraft = up elevator with each aileron command, requiring a manual down elevator command to cancel it out, which means a lot of extra mental workload for the pilot, but in the end zero differential. Actually, a high-mounted prop would tend to increase dihedral effect. The motor pylon will also act like a vertical fin, which itself has a dihedral effect (which is why the F-104 had anhedral in the wings). Might be a good idea to experiment with a few small FF gliders of the concept before investing in the R/C model.
 Jun 22, 2012, 06:31 PM Registered User im thinking of high up weight because of twin pontoon catamaran design cockpit will be above wing plus weight of motor above cockpit which with a straight wing would need lots of dihedral to keep it rightside up. with all that sweep im thinking much less dihedral would be needed. i would mount the batt high to simulate a heavy engine as in a manned with a piston engine.
 Jun 23, 2012, 10:20 AM Registered User The motor may be up high, but that weight does not act by itself. Where is the vertical location of the overall C/G including the entire airplane? Unless the motor pylon is on a hinge and free to flop from one side to the other, the fact that it happens to be up high does increase the moment of inertia about all three control axes, but probably does not pull the overall c/g up very much higher (and therefore is not a strong influence on roll stability), unless the weight of the entire rest of the plane is tiny compared to the weight of the motor. OTOH, the aerodynamic effects of the pylon and the effects of the high thrust line are likely to be much more important.
 Jun 23, 2012, 07:20 PM Registered User a two seater engine and occupants would be approx 40% of total weight. that should bring the cg up pretty high. the pylon would be the cockpit/cabin with engine mounted above and forward in tractor config another question i noticed so many sailplanes have T tails. i would have thought a low mounted stab and elevator would be more structurally efficient and lighter
 Jun 24, 2012, 07:16 AM Registered User You are correct.
Jun 24, 2012, 09:59 AM
Registered User
Quote:
 Originally Posted by captarmour a two seater engine and occupants would be approx 40% of total weight. that should bring the cg up pretty high. the pylon would be the cockpit/cabin with engine mounted above and forward in tractor config
OK, 40% 0f the aircraft weight is in the motor pod + cockpit assembly (seems very high unless you are putting the battery up there as well, which is not exactly a great idea, but we will go with it). That means that the rest of the airplane accounts for 60%, which means that the C/G is 40% of the distance from C/G of the rest of the plane to the C/G of the motor pod assembly (in other words, closer to the rest of the plane). Furthermore, the C/G of the motor pod assembly is not all the way at the top, or in line with the prop shaft, it's going to be somewhat below that. The net result is that the overall C/G will probably be only around the top of the fuselage or so. The aerodyamic effects of the motor pod and cockpit assembly will be more important than their effect on the vertical C/G location.

Quote:
 another question i noticed so many sailplanes have T tails. i would have thought a low mounted stab and elevator would be more structurally efficient and lighter
Supposedly that puts the stab above the wing wake, in supposedly "cleaner" air which makes it possible to get away with a smaller stab.

HOWEVER, the extent to which this is actually true is debatable. For models in particular, it does not seem to be that valid a theory, and even in full scale the planes that take this approach tend to have marginal pitch stability and touchy pitch control behavior. In a model, this sort of degradation in pitch stability and control carries penalties of its own, greater than what a manned aircraft would see because our capability to precisely control pitch is not as good as what an aircraft with the pilot actually in the cockpit could do.

In off-airport landings, a T-tail on a full-scale aircraft does have an advantage as far as avoiding leading edge damage. However, the tail cone and the fin both need to be beefed up significantly to handle the mass of the horizontal tail up on top of the fin.

This is an even bigger factor for models, where leading edge damage is less of an issue, but the bending and torsion loads on the tailboom in a typical contest landing (which in a full scale aircraft would qualify as an outright crash, with significant airframe damage in most cases) or a ground loop are much higher priorities. Thus, a V-tail (which keeps its mass lower, closer to the tail boom, does better on models.
 Jun 24, 2012, 07:14 PM Registered User The Let 13 Blanik had a conventional cruciform tail, like most older sailplanes, but the tail surfaces had dihedral inbuilt to keep them away from the tall grass in a landing. It was also a "taildragger" design, landing with the tail held relatively high. I think most modern sailplanes use a T tail because modern construction techniques make it possible to build them light enough, they are easier to disassemble to load in a trailer, and the tail is less likely to be ripped off by tall grass.Also, a T tail reduces the number of aerodynamic surface interfaces, allowing the tail boom to transition smoothly in the tailfin, and acting as a tip plate for the fin. It's mostly a matter of fashion, though.
 Jun 26, 2012, 01:31 PM Registered User Thanks a mill for all the insights. I'm comparing the theory with what I see in piper and Cessna singles. Imagine the piper warrior with the engine mounted way up above the cockpit, it guess like a lake amphibian. To get the same stability we would need a bit more dihedral. The Cessna singles even with high wings have dihedral! I always wondered why with engine and payload so low. The new Boeing 787 with so much sweep in flight, when the wing flexes under load has quite a lot of dihedral. I'm not discounting what you r saying just comparing with what I see.