Nose moments on CLPA ships.... the sleepless nights they've caused me.
Why are CLPA airframes saddled with such a proboscis? That thing sticking out there just looks weird sometimes and makes aero engineers thinking about mission segments cringe.
My take? The conventional wisdom is to place the spray bar 1/2" above the engine centerline. That meant a fuel tank had to be 1" thick. The paradigm in stunt was that the fuel tank had to fit between fuselage doublers which meant it had to be the width of a .35 plus 2 x (3/8"), the typical engine mounting rail that was produced in rock maple. That dictated a long tank. So you make the nose long enough to
1) fit the tank
2) fit the plumbing between said tank and engine
3) fit the engine
4) Add 1/2" - 3/4" for a shaft extension so you can sexy up the cowl.
And thus, the classic CLPA long nose moment was born. Not of aerodynamic tuning, but of necessity given the equipment available during the nascent period of stunt ships. Once it worked, it became part of the tribal knowledge of how to build a NATs winning stunt ship. To this day, we see articles written on how to design a stunt ship by using the crutch of a proven design and so it continues.
Now to some aero considerations. There will actually be a an increase in *aerodynamic* pitching moment for a given control input as the nose lengthens. It's a real effect and it's accounted for in full-size aircraft design. It has to do with the flow field around the airframe. On an inside turn there's an upwash in front of the wing and downwash behind it. The upwash will push the nose up and the downwash the tail down. It's a quantifiable effect and the formulas based on work by Max Munk et. al., are printed in most stability and control engineering textbooks.
But - and there's always a but - it doesn't come for free. Igor and a couple others are likely reaching for their keyboards as they read. The "but" is that the longer nose increases the moment of inertia about the pith axis. So the distribution of mass is now working against you, i.e, the airframe "resists" being pitched due to its increase in moment of inertia. I've never put numbers into a calculator but it would seem that the MOI is the dominant coefficient in the equation.
As to why a longer nose "grooves" better - there
may be some aero-thruth to that. It could be due to something we call "inertia coupling" alluded to earlier in the thread as the"barbell effect". Basically it's about the mass being spun around an axis, in this case the vertical axis. Without a lot of math, if you hinged a barbell at it's mid point and spun it so the weights are going around in a circle they will tend to position themselves as far from the center of rotation as possible (for all you old guys, picture a centrifugal governor). in the case of a CLPA airframe - or any aircraft - it's important to note the center of rotation isn't the center of the circle it's the center of mass of the airplane. There's a lot a weird vectors in stability and control! Long-winded, but basically the argument can be made that the inertia coupling stabilizes the pitch axis in level flight due to the constant yaw. Enough to feel? Meh, I doubt it but it is there. Our rotation rate is pretty darned low. The only time inertia coupling in pitch seems to be a concern is in spin recovery of jets.
Back to reality - I often muse about building a short-nosed CLPA ship. It should corner better. It should look like a real airplane. If we simply ditch the idea that the fuselage needs to be as narrow as possible and that the tank needs to fit over the mounting rails - the sky's the limit. Back "in the day" we were saddled by the powerplants we had available. Not anymore. Thrust to weight isn't as big a concern now so hopefully we'll start to see some diversity. I can't speak for everyone, but after 40 years over oversized Noblers dominating I'd like to see something new.
Maybe a 800 in^2 Nemesis with landing gear and a cockpit with a piped .77. Should make the last turn of the hourglass child's play.
Interesting stuff everyone.
All of the above is IMHO and feel free to offer peer review.
Chuck