I have been trying to get around to asking this myself. There have been many designs over the years that have these types of "balanced " control surfaces, with the SIG Chipmunk being one of them. Back when I built one during my intermediate class days, it was suggested that they be eliminated and just go with a straight hinge line. I don't know if I have the "feel" that some of you have for subtle control differences. I think I would need several models all lined up in a row with different set ups and fly them ion sequence and hope I could tell the difference!!
On some models, we experience a difference in how a model turns inside and outside and usually address this with line offset from center on the handle. Could the flaps be contributing to this effect? I built two Jetco Shark 45 models about 10 years apart. Both were just copies of the Jetco kit. Both were as close to being an exact copy of each other as I could get. The second model was just a bit lighter and both models powered by the same engine, the ST. G-51. But both models, to me, flew almost exactly the same. The second model had it's maiden flights at Tucson at VSC in 2005. Bob McDonald was at the grass circles with me, and eyeballed up right and inverted flight for me for a few laps and gave me a thumbs up, so I went tight into the pattern with complete confidence and had no problems. I think I was even using the same lines and handle as the first model. The main issue I had with both airplanes was that they turned outside much easier than inside. I tried all the usual trimming options on both. Both models use Dave Brown tube type push rods and I have never had any problems with these flexing in use before. The Jetco Shark does seem to have quite a bit of spacing on the thrust line, wing centerline, and elevator centerline. I just eventually thought that this might be the factor on this flying trait, and the turn rate is the nature of the beast. Would a spade located on one side or the other of the flap have any effect on how a model turns? Could the flaps be loaded more or less in direction than the other? I couldn't think of any other way to ask the question.
Type at you later,
Dan McEntee
There's a lot to digest in this question Dan. Here's some of the problem in your experience as I see it. Two different airplanes are exactly that two different airplanes totally independent of the design. When they are of one design they are still different. I built lots of F1C's in a from that made them super consistent but the resulting models did perform better or worse. They all would pretty much fly correctly and only took a few trim flights to make them ready for competition.
Removing the horn from the elevator on the Chipmunk I can't truly comment on specifically as I don't have experience with them. However, we're discussing the total hinge moment when we're talking about the CLPA. The flaps are two, three times the area of the elevator and much larger chord. This means they are the primary contributor to control moments. Removing a couple square inches from the elevator isn't likely going to make much difference in total control moments. This is why I specifically focused on the flaps. The current spades are about 7 square inches each. Not small by any means and definitely make a difference. The only question I have is how much is too much? Stay tuned for that one.
So, in terms of A-B testing removing all of the variables as much as can be done is desirable. This makes this spade approach I have a very good candidate for answering questions like this one. Fly with and without and see what happens. I do allot of that. Way more than I should prolly.
To address your specific question I don't have one. But lets review.
he Jetco Shark does seem to have quite a bit of spacing on the thrust line, wing centerline, and elevator centerline. I just eventually thought that this might be the factor on this flying trait, and the turn rate is the nature of the beast. Would a spade located on one side or the other of the flap have any effect on how a model turns?
Done correctly no, they shouldn't. This is a place to point out that they can negatively impact the trim if not properly aligned. This is why I made my first test flights with 60 second flight times. I was fortunate and got it right first effort. They can be shimmed to get the alignment better. When they don't align properly the result is to cause a static deflection until the forces are in balance. That's to say that if the spade isn't align it will cause the flap to want to fly at some non neutral position. This won't change the ultimate maximum deflection and won't change the turning radius. The only thing they do really do is change how much force it takes to get to max deflection.
Could the flaps be loaded more or less in direction than the other? I couldn't think of any other way to ask the question.
I think the above answer covers this.
As for why the airplane turns better one way than the other I think you already have a handle on this. It's a matter of understanding the moments around the CG. The high thrust line has to be compensated by down ward trim from the elevator. If the elevator has uniform deflection, it will have less power to drive the wing AOA in the positive direction and more power to drive in the negative direction. The vertical displacement of the thrust line is a "desirable" feature to counter the precession of the propeller. A little is good. A lot, not so much. How much, I don't know, buy some Paul Walker Impact plans and copy what he did.
I don't know what my basic opinion is on this one and my experience is that I lean more on symmetry than on balance by asymmetry. The later is always different the former is always the same. The vertical offset, when done properly removes some of the trim force to fly upright and balances this when inverted. The trouble is that it is power and propeller inertia specific. Change one the balance is no longer balanced For me, I prefer x lbs upright stick force balanced with x lbs inverted. That is generally always the case 0 offset engine wise.
The horizontal offset is another issue and may impact the turning radius as well. This one is a necessary evil. The wing changes the departing airflow, wing wake, which changes the AOA of the tail plane. Without enough separation the wing wake will change the AOA of the tail continuously resulting in hunting. Some can help this. but the trade off is that in some maneuvers the wing impacts is greater than others so insides will suffer while outsides benefit form the wing wake. I think Igor did a lot of analysis on this and a good resource is Frank Zaics book, Circular Airflow. Frank did a good job of discussing the impact of the change in flight path on the surface AOA and included some of the wing wake calculations.
Bottom line is I don't think spades will "fix" these. If'n it were me and I thought it was cool airplane, I'd just fly it and be happy. But then again, I like riding scooters too.