Pitch speed and 3D stability
 

I've heard bits and pieces of this thought process, and would like to learn more, without having to sit through 4 years of aerodynamics schooling.

I've been flying minumal pitch, and maximized diameter for quite a while, with mixed success. Some folks are now saying that greater pitch can help to stabilize some planes in 3D AOAs. The higher speed slipstream is better for stabilizing the tail group??? Instead of always looking for the maximum diameter, and almost no pitch, maybe I should rethink. I usually get one prop for each engine, and would like to learn a few tips for tuning my prop selection for a plane, based on this thought process.

I'm flying mostly 4 stroke glow. Maybe the same thing transitions to gas or electric, just starting with a higher pitch?

Anyone want to start off? Anyone???

RE: Pitch speed and 3D stability
 

The easiest thing to do it probably trial and error. What are you currently running?

For example - not really a good 3D example but an example none the less;

I have a Magnum .91 on a Spacewalker II - with a 14x8 I get great snap rolls and great all around performance. However with a 16x4W I don't get snaprolls that are as tight (by spacewalker standards) but the airplane will hang on the prop for a good long time which it won't do with the 14x8. It'll also tractor straight up at a very constant speed until I'm ready to stop. With a 14x8 I still get great vertical but the speed is much more than with the 16x4.

RE: Pitch speed and 3D stability
 

I'm not looking for a specific set up, just clues as to what to look for to determine if an instability might be a shortcoming of the airframe, or just poor prop selection.

Yes, trial and error will work, but I'd really like to avoid carrying 4 or 5 props per plane to every maiden flight. I'm sure that's what the "big boys" do, but many of them have prop manufacturers backing them. I pay retail, and sometimes plus shipping too...[&o]

RE: Pitch speed and 3D stability
 

I think its got more to do with the airframe rather than anything else. Consider this..

You got a low pitch prop and are into a harrier. You have to burst the throttle quite a lot to maintain height and also when things go out of shape to increase airflow over the tail. This lower pitch prop will require more rpm to move the air quicker (although the larger diameter will move more air) so the bursts will go "deeper" into the torque curve.

Now although this seems to be the preferred mode of operation during 3D, one also has to consider the destabilising effects of these torque bursts on the stalled wing. The lower prop pitch might in the end result in more wing rock (due to the above effect) than a higher pitch prop which will maintain more positive tail control (higher airspeed) with "shallower" bursts. This will be mopre pronounced with a 4st engine, of course.

I have personally experienced this effect with some planes and decided against shallow pitch props (x4). I go for x6 instead (120 size freestyle ships).

P.S. The same antithesis goes for wing loading. Contrary to popular belief, I have had heavier-loaded planes with better stalled behaviour than lighter-loaded planes (i.e. wing rock)...!