gboulton
Posts: 1862
Joined: 5/28/2005
From: La Vergne,
TN, USA
Status: offline
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An explanation I just tossed up in another forum leads me to wander in here, and provide a similar analogy....
quote:
I was wondering what the effects are from having the CG to far forward and to far back.
Let's use the tried and true teeter-totter (or seesaw if you prefer) analogy.
Your airplane is, essentially, a seesaw. Its current actual CG, or balance point, is the same as the fulcrum, or balance point, of the see saw. Just like the see saw at the park, the CG is the point at which the airplane has equal "moments" on each side. Note, this is ot the same as having equal WEIGHT on each side.
Take the seesaw. Can you balance it with a little kid and big kid? Sure. Just put the little kid farther away from the middle than the big kid. Their "moments" will be their weight times their distance from the center. So, 50 lb kid 5' away has a 250 ftlb moment, 100 lb kid 2.5 ft way also has a 250 ftlb moment, so they balance. Yay.
Now, here's the thing. Your airplane has, at its very end, a device for moving the tail up and down...the elevator. Let's say that our balance point..our CG...is a LONG way away from that elevator. That is, very far forward. You have, essentially, a tiny little 10 lb kid 25 ft away from the center of the seesaw, and, say, a 50 lb kid only 5 ft away on the other side.
What happens?
Well, you can move that little 10 lb kid (remember, he's the "tail" of our airplane) a LONG way up and down...and the bigger kid (the nose of our airplane) will barely notice, since he's close to the balance point. So, your elevator moves the airplane's tail a LARGE amount...and there's VERY little change to the airplane's pitch. in short, a very ineffective elevator.
Now...this airplane is probably flyable...it's not going to do any "jerky" or "sudden" pitch changes to be sure...but you'll need to use a whole bunch of elevator to make just a little pitch change...hence, a "nose heavy airplane often flies poorly".
Now, let's reverse this...and say that the CG is very CLOSE to our tail...which would be a tail-heavy airplane.
Envision that the airplane is moving the other direction now...the 50 lb kid 5 ft from the CG is the tail, the 10 lb kid 25 feet away is the nose...
Move the 50 lb kid up or down just a TINY bit, and the little kid on the other end (the nose) is sent rocketing into space...or, alternately, slammed to the ground.
In this case, the tiniest bit of pitch change results in a HUGE change to the nose, and thus pitch. Just a touch of elevator, and the plane is zooming straight up...which, of course, the pilot immediately "corrects" for by applying down elevator...and getting the same effect, over-control, and slamming the poor 10 lb infant to the ground. Next thing you know, you've got a panicky mother, you've ruined some parenting group's play date, and there's the inevitable lawsuit to deal with.
Hence, tail heavy planes fly only once. 
quote:
do people adjust it for different style of flying or is it something that is always a constant?
it's absolutely something that's adjusted by different folks for different tastes/preferences.
Within reason, of course, there's not really a "This plane won't FLY because its CG is too ________." issue here. (Ok, sure, put the CG at, say, the tail fin, it ain't gonna fly...but I said within reason *heh*) The manufacturer will, after testing (one would hope), determine a "range" for the CG...they'll say "Hey...balance your airplane between here and there, and the majority of pilots will find it quite controllable for initial flights".
But that recommendation is just that...it's a recommendation for a MAJORITY of pilots, for INITIAL flights.
Once you're beyond that...hey, whatever floats your boat...or, in this case, your airplane.
Some folks (me, for example), like what others would describe as a HYPER sensitive elevator. I want my airplanes to come just shy of a high speed stall if I so much as THINK about pulling back on the stick. So, I fly with the CG "back"...usually either at the rear of the suggested range, or even behind it. That doesn't make me a "better" pilot or anything...it's just what I prefer, and I'm not alone in this.
On the other hand, some guys like a very "squishy" elevator...so "squishy" that the majority of folks would think something was wrong, and some of us would almost wonder if the elevator's even there! They fly with CG's well to the forward end of the suggested range, or even in front of it. Again, these aren't "good" or "bad" pilots..it's just what they prefer.
In either event, it IS safe to say that the manufacturers are usually right...something within their range will prove flyable for a majority of pilots during initial flight testing...and then they can move the CG from there as it suits them. Over time, if you find that you happen to prefer planes "a bit nose heavy" or "a bit tail heavy", then you can "cheat" a little, and balance the plane toward one end or the other of the suggested range initially.
HTH!
_____________________________
The free man will ask neither what his country can do for him nor what he can do for his country. - Milton Friedman
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