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Old 03-02-2008 | 04:41 PM
  #17  
da Rock
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From: Near Pfafftown NC
Default RE: Stabalizers and elevators


ORIGINAL: David Ingham

I have started reading Basics of Model Aircraft Design by Andy Lennon. It is full of practical information, but so far not technical enough to make interesting reading for me.
I found one thing that seems to be an omission or error. On the first page of Chapter Six he said:
"However, at the forward CG, the model's longitudinal stability would be high, and it would recover by itself from any pitch disturbance, returning to level flight. It would be easy to fly, but not highly maneuverable. Moving the CG rearward improves maneuverability but reduces pitch stability."

What is missing is dynamic stability of the phugoid path. As clearly explained by Charles Hampson Grant (before he made a mess of Dutch roll stability), an airplane with the center of gravity too far forward, for its length, tends to dive and sour with increasing amplitude, ending in a crash loop or stall, unless corrected by the pilot. So a model with the center of gravity too far forward is not generally easy to fly, especially if it is slow. Admittedly, this is less important with RC than with free flight, but "dynamic" stability is a requirement for civil piloted aircraft.
What you are talking about is certainly true, but Lennon says, "the forward CG", nothing about a CG that's too far forward. Matter of fact, doesn't he talk about dynamic stability and certainly does mention CG range in a number of places.

No matter, what you mention should be considered by everyone.