I have been lead to believe that there is one speed that a given plane will fly for any one AOA. Add throttle, the nose rises and the plane climbs at (idealy) the same speed. Decrease throttle, the nose drops, the plane decends at that same speed. All of this with no trim changes. But in reading several of these threads re: thrust vectors, CG , AI, etc. adjustments, the plane should trim out to fly straight and level no matter what throttle setting (at least from 1/2 to full). Sounds like there is a conflict here. I've been caught off base before.

Please insert AOA everywhere it says AOI (with one exception). I had a brain f*rt.

Actually, you can do all that yourself and it'd help us understand what you're asking. There is an edit button on the upper right of each post of yours.

Trim speed depends on weight, airspeed and air density (area being constant).
For any trimmed condition, adding power climbs, decreasing power descends.
As weight is burned off, the plane will climb at the trimmed condition.
To maintain altitude, the trim will change as weight is burned off.

What Tall Paul says is right on the money. Any light plane that I have flown ,that is the way things work. Once you get to the altitude you desire you level off and adjust your throttle and trim it for hands off level flight at your desired speed. If you give it more power you start to climb ,given things being equal. Every thing effects how your plane acts,how much you weigh,air temp ,and so on. Now,I am wondering just how a full aerobatic plane would act. I am thinking it would still respond to power change,etc., but not so touchy about it.

If the thrust angle is off the plane will tend to change pitch with changes in throttle. Too far down, and the plane will lift its nose when you let off and drop its nose when you apply power. Too far up, and the opposite happens. When the thrust angle is just right, the planes pitch sensitivity to speed is minimized. Proper thrust angle changes with changes in CG and wing/tail incidence also, so those need to be set right before tuning the thrust angle.

If you decide to build the plane with less down-thrust the main risk is it might make the plane want to gain altitude when you advance the throttle after trimming for level flight. Can these two quotes be reconciled or am I comparing apples and oranges? What happens to one AOA one speed?

You're close to right here. But it's really the wing to tail relationship that trims to that AOA. But as long as it can hold that AOA you're dead right with this first part. Reduce the AOA of the wing and the plane needs to fly faster to compensate to hold altitude. If you don't add power the plane will descend increasingly if you hold the attitude until the uprising air meeting the wing is at the trimmed AOA.

You're describing what a lot of flyers THINK the plane should fly like and they'll go to great lengths to trick it into flying like this. Adding lots of downthrust is one answer so it combats the plane's natural tendency to lift the nose with additional airspeed. Generally you'll find that such people are those that have not flown in real planes or have only flown "simulators" and feel the need to "fix" their models so that they fly like they think they should fly rather than adapt to how they really fly.

Or if you're working with a hot aerobatic model then you'll be operating with the CG close to or even at the neutral point of the model. Such a setup requires extremely little or no "up" trim of the elevator compared to the wing. And when there's no "up" trim because the CG is so far back that the model doesn't want to fall into a dive on its own without some uptrim to hold the nose up then the model will not show a tendency to climb with increases in power or dive with decreases in power. But this last situation is an extreme case.

Normally we use SOME downthrust to limit but not totally eliminate the natural tendency to climb with added power. When set up this way the model is still fine to fly and flies in a way that most of the full sized planes fly.

Could one say, that a scale model built to the same geometry as the full scale subject will demonstrate the same flight charactoristics ? And the other adjustments are to lighten the pilot's work load and make up for the inability to " feel " the plane in flight?

No. The wingloading won't be scale.
And actually impossible to achieve.
Models are lighter, and overpowered relative to full scales.
The characteristics are the same... climb with power, stall when too slow.

And not only is it impossible to match the wing loading and power loading, but the Reynolds effect messes up things about the individual parts that the whole geometry isn't in sync either.

Too many differences.

Please bear with me. I am trying to get a grasp of what can be preserved and what must be changed when scaling down any airplane. DO things like : thrust line; CG location; CD location; CL location; decalage; tail moments; things like that. As a wing section is scaled down, say a Clark Y of 5' chord down, do the RNs change liniarly or exponentialy? Can you say there is a direct or inverse relationship of that curve to those components that must be changed? I have a feeling there are too many interrelationships to attempt to make it simple.

Reynolds Number is a scaling factor that allows engineers and scientists to make comparisons between experiments involving fluid media like air and water, large objects with small, etc. The definition for Reynolds Number is Re = (Length x Velocity x Density) / Viscosity. When comparing a full size airplane with a model airplane the air density and air viscosity are the same so they become irrelevant. Whats left then is a ratio between chord lengths and velocities.

As an example a full scale monoplane with a cruise speed of 200mph (293 fps) and a 5' chord would have an Re = 6363 x 293 x 5 = 9,322k. A 1/5 model of the plane with a 1' chord and 40mph cruise (59 fps) would have an Re = 6363 x 59 x 1 = 375k. The models Re is only 4% (1/25) of the full scale plane, not 20% (1/5) if it were a linear relationship.

Just for fun, how fast would the model need to fly to have the same Re as the full scale? V = 9,322,000/ (6363 X 1) = 1,465 fps! Thats nearly 1,000 mph!

You can get a really good grasp by studying scale models that have been made for competition. For the most part, scale models fly fairly well. They do not fly with the same characteristics as their fullscale counterparts because as mentioned earlier, wing and power loading cannot possibly be duplicated. They do fly but pretty much as any other model that has the same areas and power and weight would. What has proven to be a predictable characteristic is that when the models are truly "model size", not say a 30% for example, is that pitch stability suffers. It's been an accepted situation since forever, that a scale size horizontal tail may need to be enlarged. Most scale builders do not wish to lengthen the aft fuselage when increasing tail area will do the same. It's easier to spot a disproportionately long fuselage than a tail that's been enlarged a little.

For years, it has been fairly common to read the build article for a scale model that mentions that the designer increased the tail area 15% "so it will fly safely". No other special considerations have been identified as needed by our models. Moreover, it's an absolutely safe practice to work out incidences and moments with the identical formulas that the aeronautic industry uses when designing a model. You see exactly that at http://www.geistware.com/rcmodeling/cg_super_calc.htm That is an application for modelers that is based entirely on the same science used by the aero industry. It works for them and works for us.

You're not facing any issues that the rest of us didn't face.

The biggest is that the full scale arrangement of thrust line, stability margin and wing and tail incidence is set up for a completely different balance than we would have on a scale model of the same plane. Due to the lighter weight and Reynolds number effects they share nothing at all in common other than the exterior shape. Often we won't even get the best performance by using the scale airfoil depending on the full size design.

Also in your first post you more or less hit the nail on the head as we sort of described along the way. Full sized pilots fly their planes in a far different manner than we do as modelers. For them the designed in safety factor demands a more forward CG and that comes with a negative load at the tail vs our models where there is often a neutral or even positive lift load at the tail thanks to our "tolerance" of more rearward CG's compared to most full sized airplanes.

Why the difference? Because full sized pilots work with the airplane more then most modelers are willing to do from the evidence of the threads here about "why does my plane nose up?". Full sized pilots use the throttle to control climb and the elevator and elevator trim to control speed. When was the last time you found a modeler that flew like that? Yeah, I thought so.... Full sized pilots fly that way because of the mandated in stability margin in all but the hot competition aerobatic ships. With that stability margin comes the need to work with the aircraft rather than just horse it around.

But even the scale model fliers won't tolerate or learn to use much of what the full sized pilots will. So we are back to rolling our own when it comes to the thrust, wing and tail setup on a scale model.

Does that help you settle the confict?

I have designed several scale models and since I am a chemical engineer, I don't (can't) get very deep into the aeronautical engineering. I have had a reasonable amount of success designing on a "don't do anything really stupid" basis. For a very serious competition scale model I suspect you will find that the thrust line and incidences match the full-scale plane. If they don't the outline deviations will be very obvious. I also suspect static judges look for these deviations. The planes I have designed on this basis have all flown. Not necessarily "on rails" like a pattern ship, but they met the requirements of scale flight.
Chuck

Apparently the subject to be scaled supplies only the profile, and the modeler "designs" a model that best fits that profile. And make it light, light, light. I think I'm in the ballpark now. Gentlemen, thank you for your patience.

YES!
If you look at the e-Flite T-28, which is surely the best scale model ever offered, the reason for its success is it's made of foam, and is incredibly light.
The full-scale T-28 has two "features" that make it a poor choice for a model.
Wing area and fuselage size.
If the model is rendered in conventional balsa-plywood construction, the resultant wingloading makes the plane a handful.. Takeoff speeds are high, as are lainding speeds, and these planes seldom survive long.
The e-Flite version, being as light as it is, is superb!
It's almost of trainer quality handling, but put in a slightly larger motor.. Power 10, and it will go vertical! NO T-28s can do that!

As to the climb-dive issure. The faster the airflow over a wing, all else being held constant, the more lift generated, therefore a climb. The slower the airflow, the less lift, and therefore a descent.

Actually, in the 1970's Dave Platt Models had a .60-size T-28 that was all-balsa, and it flew very nicely, even when equipped with an interior and retracts. The wing area wasn't too small, and the large fuselage didn't cause any particular problems. The plane took off and landed at very reasonable speeds. If the modeler could handle the then-current crop of "Pattern"-type airplanes, they could certainly handle that one. It was considered an excellent model for fliers just getting into scale models..

Has anyone tried Dave Platt's "Scratch This" series videos? Any comments?

There are reviews of the Plattvideos on www.rcscalebuilder.com underthe Dave Platt forum

Some aviation eras scale better than others. As engine power density and aerodynamic science improved, full size planes became faster with higher wing loads - both detrimental to closing the the Reynolds number gap.

The World War I era offers the scale modeler an opportunity to have the best of both worlds. The full size planes of the era were underpowered and over-winged, yet the basic theory of flight control was advanced enough that the models respond predictably to pilot inputs. R/C planes are pretty easy to overpower so flying the model a bit fast on a slightly tweaked airfoil gets you about as close to the real thing as you can get in my opinion. The full size planes were constructed largely of wood and fabric which is an added plus for those wishing for the ultimate in scale fidelity.

Balsa USA offer some nice kits of subjects from this era and although they take a few too many liberties, no one I've talked to has a bad word to say about their flying habits. Proctor Enterprises make some of the nicest R/C kits ever made including scale airfoils, machined turnbuckles and even warping wings in the case of the Fokker Eindecker. They fly great too!