Best tailplane

I must have been drunk - I have absolutely no recollection of posting this. :-)))

Malcolm

I'll stick with the standard set up of horizontal stab and elevator for sub-sonic model flight.

Ed Cregger

"malcolm fisher" wrote in news:0qJMe.11594 $ snipped-for-privacy@newsfe4-win.ntli.net:

In my experience, which includes something like 100 stabs and maybe 3 all- moving tailplanes, the all-moving tailplanes give smoother control and have a better feel to them. However, I'm not sure they're usually worth the extra work (well, sometimes it's extra work), and I suspect that they're much more susceptible to control slop problems.

What kind of model? For a big sailplane, the all-moving tailplane should have significantly less drag (both parasite and trim drag) than an elevator/stabilizer combination, so it probably makes a lot of sense there.

That is very interesting.... I haven't flown a model with an all-moving horizontal stab, but have flow a Cessna 177 (Cardinal I think), which is of that design. Of the planes I've flown the C177 was the smoothest and most responsive. It spoiled me.

According to an article in one of the recent "Quiet Flyer" issues, there's no difference in drag between the two. Actually, they claim that in transition a correctly implemented stab+elevator configuration produces _less_ drag than full-flying tailplane (although the difference is negligible).

The above, of course, assumes that the stab is installed properly, i.e. that the angle of incidence is chosen so that there's no need to trim the plane with elevator. If it isn't so, then the elevator will have to be trimmed, meaning that it will introduce some unwanted drag. Full-flying tailplane, on the other hand, naturally supports the adjustment of its angle of incidence without introducing any parasitic drag.

-- Best regards, Andrey

Andrey Tarasevich wrote in news: snipped-for-privacy@news.supernews.com:

I'll have to go back and read that article. I agree that if you have the stab mounted at the right incidence, then there's no trim drag difference between the two. However, I'd think unless you got the elevator/stab seam to be perfect on top & bottom, the seam would create parasite drag. It would normally be a pretty small effect - certainly a couple orders of magnitude less important than picking a good airfoil & planform for the wing. Offhand, the only way I see to get lower drag on the stab/el is to have the seam trip the flow, but that doesn't sound like a very good way to trip, and I'm not even sure you'd want to trip the flow on an elevator when it's flying at near-0 angle of attack.

Okay, I *really* need to go back and read that article - I'll hold off on speculating further.

I'm sure I read that article in June 2005 issue

or

Full-flying tailplanes typically are used on faster aircraft because they have less drag. They are always streamlined with the airflow, have no elevator/stabilizer gap, and have plenty of authority at low speeds. Stabilizer/elevator combinations create drag in cruise since the elevator is normally trimmed down somewhat. The stabilizer is set at a compromized angle so that it helps keep the nose up in climb or glide, and as cruise speed builds the elevator must go down to counter the increasing nose-up force. The resulting angle between the stab and elevator creates unnecessary drag. There's a hybrid system used on airplanes like the Cessna

180/185 and the fabric-covered Pipers. Trim is accomplished by moving the leading edge of the stabilizer up or down, and the elevator will be streamlined behind it whenever the airplane is flown hands-off.

Dan

Paul Ryan wrote in news:Km2Pe.7572$A% snipped-for-privacy@newssvr13.news.prodigy.com:

Generally agree - certainly, if you're building to a reasonable weight, it's very difficult to make a stabilator anywhere near as strong as a stab/el. For violent aerobatics, I'd agree that the stabilator is a distant second.

Yeah, two related problems there. First, it's hard to get near-zero slop in a stabilator, and second, because you deflect a stabilator only about half as much as an elevator, the stabilators tend to be more sensive to slop. On the plus side, the servo loads can be much smaller. Although, offhand, I don't think I've ever had a problem with servo loads on an elevator.

True, though for casual building I think it's easier to get a decent airfoil in a stabilator than in a stab/el, and it should have lower drag at low AOA- that's what thin symmetrical sections are good for. For a saiplane or an airliner, the stab should pretty much always be at low AOA, and drag is a big deal for those planes, so I think stabilators make sense there. For your aerobat, where drag isn't a big deal but you need big forces (read "big Cl") out of your stab, the stab/el will, again, be much better suited.

heh you haven't seen some of my electrics

Bravo on the tapered spar caps. I've only done that once or twice, and then only on the wing spar, but it's one of the few structure weight savings that doesn't weaken the structure or introduce new failure modes.

I'd say that in essence, the stabilator can have as good (usually better) fixed-camber airfoil but, as you point out, it can't vary it. As above, if you're normally using your stab over a narrow range of Cl's around zero, the fixed airfoil is just fine. If you want big forces, then you'll want big camber, which gets you back to the stab/el.

Not sure I agree there. First, if you need to make trim adjustments, the cambered stab/el combo is probably going to be draggier than a stabilator that's simply dialed into the right AOA. Bear in mind that, for example, a stab that's built at -2 deg when it should be at zero will wind up being trimmed so that the LE is still at -2 deg (because you can't change that) and the elevator is at +2 deg, so you get the profile drag of a crudely cambered airfoil without any of the CL advantage.

Second, if you're doing heavy maneuvers, the local airflow may be far away from the reference datum or whatever your stab was originally aligned to, so the LE of the stab won't necessarily be lined up with it. Actually, it's not clear to me that for big tail loads, having the front of the stab parallel to the local flow is actually the ideal.

I agree that that's the best way to build a stab/el hingeline, but I usually revert to simpler structures. I suspect that you build much larger planes than I do.

Look: you have to counter the airfoil's natural nose down pitching moment (torque) one way or another-, it doesn't really matter which type- full flying or hinged- stab you have, and there will be some drag involved. If you want max efficiency- you have to move the CG back so that the stab has to make less down force, and the position of the cg counters the nose down torque of the airfoil. The point where these two forces(actually moments or torques) equalize is known as the neutral point. A plane with its cg here will be neutrally stable, and will not recover from a dive, it'll just stay where you point it. The stab will not be pulling down or up. If you move the cg farther back, the stab will actually be contributing lift to the plane, but the plane will now be unstable in pitch, and will be hard to control, if it starts into a dive, it'll pitch even farther down... Paul

Paul Ryan wrote in news:UO7Pe.626$ snipped-for-privacy@newssvr21.news.prodigy.com:

So far, so good.

The story varies somewhat with airfoil and tail volume coefficient, but the neutral point is usually a bit farther back, where the stab is at about the same Cl as the wing.

A lot of old-timers had lifting stabs, and many of them (particularly all those pylon designs) had CGs well behind the mid-point of the wing. They had to be stable because they were free flight.

In the extreme case of a canard configuration, the rear wing is actually carrying most of the weight of the airplane.

For best efficiency on a 'normal' airplane, you generally want the stab to be generating zero lift - it's just not as good at creating lift as the wing is.

You need some formal groundschool. The CG must ALWAYS be ahead of the centre of lift (or centre of pressure, CP), or the airplane will be unstable. You CANNOT have the stab generating no up or down forces in a "neutral" setting, or the airplane will climb or dive as the centre of pressure shifts with AOA changes and the pilot will be fighting to keep it level, and it will rapidly get out of control. As angle of attack increases, the CP moves forward and causes the nose to rise further, making down-elevator necessary. As the aircraft pitches down, AOA decreases and CP moves back, causing and increase in nose-down forces. Many full-scale pilots have found this out the hard way when they loaded their airplanes too far aft and crashed shortly after takeoff. They will not recover from a spin. That's why the government mandates aircraft design to have CG ahead of CP, and the pilot operating manual has charts and graphs to enable safe loading. Aircraft with CG behind the CP so that the tail has to lift are fantastically dangerous. If they stall the thing will flat-spin immediately and will not recover. Some of the early WWII aircraft were of the lifting-tail type and were hated by their pilots, being underpowered and on the edge of the stall all the time. Canards have "lifting tails" that are highly loaded to make them stall earlier than the wing, dropping the nose and regaining fliying speed, as things are supposed to do.

Dan Commercial Pilot and Instructor and Aircraft Maintenance Engineer

Dan: This is beyond the groundschool, with its simplificaions for the mathematically challenged, my friend. More like aeronautical engineering... And don't worry, I'm a pilot as well, bonanza F35, N5070B, and 1000+ hr hang glider pilot (survival alone is proof of... something). Actually, to be a little more precise, the mean aerodynamic center, or MAC, is the point about which all aerodynamic moments are balanced. The definition of stability is having the CG in front of the MAC. If the CG is at the same point as the MAC, the plane will be neutrally stable. If the CG is behind the MAC, the plane will be unstable. That's all there is to it, except finding the MAC ain't always trivial. A lot of Aerobatic pilots actually prefer planes that are unstable, they are better for doing 3-D maneuvers, but they're a little hard to land- they flare themselves with no help from the pilot... in fact, you have to keep them from doing it. What you are saying agrees with what I'm saying for the most part, whether you realize it or not. Unstable planes are hard to fly, and the ease with which they do flat spins, etc. is exactly the reason why hard core 3-D pilots prefer them set up this way. Yes, the center of pressure does move a little causing a small nose down pitching moment, but this change of moment is small compared to that contributed by the tail. The whole MAC concept is based, I believe, if memory serves, on steady state trim conditions. Remember- I'm on your side... -Paul

Dan_Thomas snipped-for-privacy@yahoo.com wrote in news: snipped-for-privacy@g44g2000cwa.googlegroups.com:

Ah, but I rigidly attach the stab to the fuselage. Thus, when the nose pitches up, the stab that was formerly generating no up or down force will start generating lift, and since it's far behind the CG, it will cause a large pitching moment, bringing the nose back down.

If the tail can stall before the wing, then yes. The other big no-no is to put the stab where it can get be blanketed by stalled air from the wing (the BD-5 springs to mind).

Then why is it okay for a canard to have the rear wing generate lift, but not for a conventional layout?

From Dan:

You meant WWI no doubt, and prior. The Bleriot monoplane would be a good case study in the lethality of lifting tails. Bill(oc)

Yup. Typo on my part.

Dan

The CG of a canard is well ahead of the aft wing's CP, so that the front wing is rather heavily loaded. The front wing also has a higher angle of incidence so it flies at a higher angle of attack and reaches stall angle sooner, preventing the main wing from stalling. The canard's appeal comes from the drag reduction since there is no downforce generated that must be made up with more lift from the wing, as in a conventional layout. Forward CG on a conventional airplane means the stab has to push down harder, and the wing has to carry the airplane plus that tail force. It's why stall speed declines a little with aft CG. The canard's drawbacks include long takeoff rolls, since that little wing can't lift the nose very soon to get the main wing flying. Another one is the huge CG shift when the occupants get out, letting some of the smaller ones fall on their tails unless the nosegear is retracted to get the weight forward some. I need to apologize for the snarky tone of my earlier post.I was irritated about something entirely unrelated and it came through in the typing. Getting old and grumpy. Dan