Hello everyone I have a question about pitch stability. Maybe this has already been discussed in detail, or is too involved, feel free to send me a note directly if you wish (seibel999 at hotmail.com)
I've been reading Andy Lennon's "Basics of R/C Model Aircraft Design" and Martin Simons' "Model Aircraft Aerodynamics". They both describe pitch stability as being determined by the position of the CG in relation to a "neutral pont" for the aircraft as a whole which is determined by the size and shape of the wing and tail in combination. Little or no mention is made of this "neutral point" moving around in response to changes in trim. Basically my question is this: does this "neutral point" move around in flight, assuming no actual flexing of the aircraft is going on? There's a cool website "See how it flies" www.monmouth.com/~jsd/how/ this author emphasizes the idea of decalage--if the horizontal tail is flying at a positive a.o.a., then it must be lower than the wings a.o.a., for the aircraft to be stable. I don't know if this is technically true 100% but that was the basic drift. In that case giving nose-down trim (with no change in weight distribution) would make the aircraft less stable, i.e. move the neutral point forward, right? So a glider could be stable at some positions of the control stick but not other more forward positions? If so I'm surprised the books I mentioned above didn't talk about this. Here's what I see with my Gentle Lady: trimmed for min sink--very stable (if you bump it into a dive, it will then pull up strongly.) trimmed not too much faster--glide isn't much steeper, appears stable, until you bump it into a steep dive, then it shows that it's only neutrally stable, no signs of pulling out. So the question again is, does it make sense to think of the "neutral point" as moving forward when the aircraft is trimmed more nose-down (with no change in the CG location)? Can this be explained with "straight" aerodynamics (no flexing)? Are changes in the downwash angle important? Is the simple idea that any reduction in decalage (any increase in the tail's positive angle-of-attack, or reduction in the tail's negative angle-of-attack) makes the aircraft less stable, a valid idea? And a logical extension--I don't know how all this relates to "tuck under"--my glider always remained fully controllable, just not always stable--if a glider becomes uncontrollable where full aft stick won't bring it out of the tuck, can that be explained through "straight" aerodynamics or does that indicate that the wing or control system must be flexing? Thanks for any input, either to the list or to me directly. seibel999 at hotmail.com _________________________________________________________________ MSN Photos is the easiest way to share and print your photos: http://photos.msn.com/support/worldwide.aspx RCSE-List facilities provided by Model Airplane News. Send "subscribe" and "unsubscribe" requests to [EMAIL PROTECTED]
