redcommander

Actually, if a non-symentrical h-stab is used (and designed in accordance with the static margin), wouldn't be possible to have an airplane maintain the same flight path angle and ifferent power settings without trimming the elevator??

redcommander

sorry, let me try again minus the typos

Actually, if a non-symentrical h-stab is used (and designed in accordance with the static margin), wouldn't it be possible to have an airplane maintain the same flight path angle at different power settings, without trimming the elevator?

rmh

Try it - on many models this has been done and depending on with whom you speak -works well or --- not at all.
there is a practical limit to the speed which this "seems" to operate in.

BMatthews

Redc, that is sort of what free flight models try to do with their lifting stabs. Well that and thanks to the stabs being positively loaded due to the rearward CG's it helps with the lift and all.

But as Dick says it only works over a fairly narrow range of speeds. I've seen models that flew fine for a few flights suddenly lawndart themselves when a gust or deeper than normal stall got them flying too fast and the stab lift overpowered the wing. I even had this happen with a rubber powered P30 design of my own. Undercambered airfoils on wing and stab. In normal flight the model was just the right amount of pitch stable and would recover nicely from mild stalls. But if it stalled deeper or a gust pushed the plane into a dive or spiral and the speed built up a little the stab overpowered the wing and it was LAWN DART TIME!..... well as least lawn dent time. A 50 gm model with a 9 inch freewheeling prop on the nose can only do so much damage after all.... Granted these models fly VERY close to the neutral points but it's a good example of how stab lift can be vairiable in it's balance with the wing. For my P30 I built a new stab with a lesser camber flat bottom stab and re-trimmed to work with the same CG and all was well in the pitch department from then on.

mesae

Possibly, over a relatively narrow speed range, like Dick said. But it wouldn't work in all attitudes, and like I wrote, it couldn't work for all power settings. And it almost certainly wouldn't make a good aerobatic airplane. I have already written that it could also be approximated with down-thrust, but that also doesn't work for all attitudes and power settings either.


edit: added "either".

mesae

This is an illustration (along with my Telemaster dive story) of why "lifting" tails, are not true lifting tails over the entire flight envelope, if we want pitch stablility over the entire flight envelope. A tail must provide negative lift over some part of the flight envelope for consistent positive static stability.

BMatthews

Well.... actually the way I see it is that it only has to provide less lift than the wing does. For CG's behind a specific point the tail has to lift positive for all flight attitudes. But when it lifts harder than the wing does then we get into trouble.

I agree that for CG's forward of and slightly behind the generally accepted 25% point the stab will operate in a negative lift mode and for CG's a short distance behind the 25% point the value will float from postive to negative depending on the airfoil and other considerations. But once we get the CG back to the 35 to 40% point it's all positive. And for most free flight contest models the CG's are at least 40 % back. The P30 in my story has about a 50% CG or thereabouts.

I'm having an awful time finding it now but Mark Drela posted a great chart that showed a continuous progression of stabilizer sizing from very small to canard and how the lift on the "stabilizer" alters for a neutral point CG from negative for small % stabilizers to strongly positive for canards (obviously) with the large stab free flight types and tandem wings shown as well. It also strongly drew home the fact that a canard is just a very unique form of normal planform as far as stability analysis goes.

rmh

many opinions are available when it comes to establishing a hands off setup
Being relatively ignorant about all the formulas which typically fly out of the dusty closets -- I have to resort to-- basic laws of nature and hopefully just simple evaluations.
The hands off trimming over various speeds and from upright to inverted -- simply escape me
I can't do it .
It is much like weaning a horse from food - I can get almost there - then he dies .
The strange part is -others simply poo poo the problem stating their models do this with impunity.
I get stuck with lift and gravity as natural enemies and so I can't resolve the problem.

BMatthews

Dick, that's why I like the simple dive test. There's no math to study when you're in the sky and the dive test takes all the factors into account in a very simple and easy to allow for way.

While I really like studying the theory you'll note that I seldom take part in the heavy math parts. While I understand the need for the math and applaud those that take this on like a favourite cross word puzzle challenge I just can't wrap my grey cell around it unless I have to and it's reasonably simple. But I sure do enjoy studying and discussing the basic principles.

mesae

This is true from a tail-lift viewpoint, except that it doesn't consider the inherent instability of a cambered wing. This instability must be overcome with a positive static margin (or at least a flyable one). There are various ways to do this. I believe these "tucks" we have been writing about have more to do with the wing's inherent instability (especially the strongly cambered FF model wings), than with tail-lift simply exceeding wing lift. It seems unlikely to me that a horiz tail that is a fraction of the area of the wing, and has a smaller incidence angle, is simply generating more lift than the wing.

As the airspeed increases (AOA decreases), the AC of a cambered wing moves aft, and if the tail (nose up) moment doesn't equal or exceed this wing-generated nose-down moment, voila, sudden nose-down pitching moment that can be difficult or impossible to recover from. Obviously these lifting tail designs can work, if we operate them in the airspeed (AOA) range in which they are stable. Trouble is, most models don't have airspeed or AOA indicators.

Canards do have the fundamental difference from aft-tail airplanes in that the canard always lifts positively, which makes them more efficient, since the total lift load is shared between the canard and the wing. Canard sizing is tricky though, because if you make the canard too big, the airplane becomes pitch-unstable by overcoming the wing's stability contribution. Remember, it's behind the CG so a canard's main wing is stabilizing for the whole bird. That's one reason canards generally have much longer takeoff runs than "equivalent" aft-tail airplanes. You can't make the canard big enough to raise the nose at a low airspeed, without making it unstable, or causing the canard to stall after the wing, which is BAD. With an aft-tail airplane, you can almost always make the horizontal tail bigger if you need more pitch authority, and it also increases stability.

BMatthews

The tail area doesn't need to have more lift than the wing. It has the leverage of the tail moment arm to help ampllify it's effect. SO it comes down to how the lift of the tail changes in a non linear manner as it did in the case of my rubber model with the tucking problem. At some point it went from a generally positive pitch stability manner to where the older strongly cambered stabilizer generated enough extra lift that it overpowered the wing. Changing it to a lower camber choice ensured that the tail lift could not achieve that level of magnitude again and so a positive, but undoubtedly variable, positive pitch stability resulted.

You're also reffering to the older concept of the AC of the wing moving about. The way I understand it these days is that the presently accepted way to deal with the pitching moment is to consider the AC as being constantly at the 25% chord point and then describing a pitching moment to deal with the torquing effect. I'm not sure when this changeover took place but the engineering types that hang around here explained this in a couple of threads a year or so ago.

mesae

The engineering book I am using is the one of, if not the newest book on the subject and covers some areas in more depth than some older books, and even has a few concepts that are not included in any other single source, like quaternion algebra for 6DOF flight sim coding, and considering the spiral component of slipstream to improve the momentum model of propeller thrust generation. I have read about mach tuck and other AC shift characteristics in other older books as well. It's not something that can be ignored in all cases, especially extreme ones.

The wing AC does actually move with changes in AOA. You no doubt increased the usable airspeed range by going to a symmetrical tail airfoil becuase you decreased the pitch-down contribution of the tail, making the AC shift on the wing less effective in pitching the nose down. Also, it's the neutral point, or AC of the whole airplane, with respect to the center of gravity, that determines pitch stability, not the CG position relative to the AC of the wing by itself. That determines tail-lift requirements. If the tail volume is large enough, the neutral point can be aft of the 50% chord point or theoreticaly almost anywhere. In fact, the larger the tail volume, the less important small changes in the wing AC become.

The quarter-chord point works well as a first approximation, and even beyond for many cases, especially for symmetrical airfoils, since they have no pitching moment about their AC. But to go beyond first approximation, especially in borderline cases like our nose-divers, we need to get closer to the truth by considering wing AC shift, and even fuselage AC, which can shift the AC of the airplane 3% MAC or more, over the wing and tail-only model. 3% MAC is significant when you consider that for full-scale at least, 5% MAC is considered the minimum acceptable static margin.

I'm not saying that tail-lift did not contribute to tuck; I believe it did. But I suggest that it might not have been the only important factor, and just because changing the tail airfoil solved the problem for the airspeeds you use, doesn't mean the problem wouldn't creep up again if you flew it faster. But it was a good solution, since as you say, the problem hasn't re-ocurred, which means the speed at which the problem would show up again is higher than encountered in actual use. All airplanes have speed limits, above or below which bad things can happen, so I'm not even suggesting that predominantly lifting tail designs are bad designs. Whatever works, I always say.

edit: I probably should have used CP instead of AC in much of the above discussion. And yes, thin airfoil theory puts the AC and CP at the quarter-chord point for incompressible flow.