When a wing has less dihedral is it more likely to stall in a steep tight turn?
From : Don Stackhouse
An airplane stalls, whether in level flight or in a turn, because the pilot pulled the nose up too far and forced the wing's angle of attack to reach the stall angle. If it's a free-flight model, then the person trimming it before flight trimmed in too much "up" and caused the nose to go up too far and the wing's angle of attack to reach the stall angle. In either case, the cause of stalls is "pilot error".
Dihedral can have a positive influence on the sink rate in a turn on airplanes whose turning radius is relatively small in comparison to their size, such as HLG's. We have found both by test and analysis that the amounts of dihedral typically used in a 2-channel HLG reduces the sink rate. This seems counterintuitive, but it works.
We first discovered this by accident many years ago while developing improved versions of our early Monarch RCHLG's. We tried one with about 2 degrees less dihedral per side, expecting the performance to improve, since after all, "everybody knows that dihedral hurts performance." To our surprise, reducing the dihedral by that small amount had little measurable effect on level flight performance, but the reduced dihedral made the sink rate in tight thermal turns get noticeably worse! Investigating further, we found parallels in the bird world, where slope soaring and high-altitude soaring birds that make big flat turns tend to have little or no dihedral, while those that work small, low altitude thermal lift tend to use lots of dihedral, and may even vary the amount depending on the radius of their turn. It's not for stability, they already have the ultimate in "fly by wire" systems to provide all the "active stabilization" they need. We've since explained it via analysis and reconfirmed it through additional experiments.
Since you're probably interested in inverted flight as well as upright, positive-G flight, for an aerobatic model you probably want little or no dihedral. This is not for performance, but rather to keep the handling as equal as possible between upright and inverted flight.
Not necessarily true. If the airplane has lots of dihedral and ailerons, it is possible to have adverse yaw from the ailerons couple with the dihedral and cancel out some or all of the ailerons' roll authority. However, the real culprit in this case is not the dihedral, but rather a lousy aileron design that makes too much drag when deflected downward, causing that adverse yaw. If the ailerons are efficient, you can have as much dihedral as you want and still have good roll response, including during inverted flight.
For example, our old Monarch 'CX' HLG had the same polyhedral setup as the 2-channel Monarch 'C', but it also had extremely wide-chord full-span flaperons, right across the polyhedral joint and all the way out to the tips. The roll response with ailerons alone, zero differential and zero rudder input was about the same (maybe even a hair quicker) than the poly 2-channel version's roll response using rudder. With rudder plus aileron, so that the rudder was creating proverse yaw to add to the ailerons' authority, the roll rate approached the "Pitts Special" category.
The key here was the wide chord of the flaperons. Many model designers go by the guidelines for aileron chord for full-scale aircraft, such as the 20-25% of chord recommended in Abbott & Von Doenhoff's "Theory of Wing Sections". For larger, faster models that have Reynolds numbers approaching those of full-scale aircraft this might work. However, for smaller models with smaller Reynolds numbers, a much wider chord aileron/flaperon with less angular deflection works much better. Once again, we have demonstrated this repeatedly through both analysis and test. In our experience, at low Reynolds numbers the best place to put the aileron hinge line is about the same place you would put a turbulator, assuming that you can sort out the structural problems involved.
Don Stackhouse
DJ Aerotech
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