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Well, I'm not going to imply that exceeding CG limits will definitely cause you to crash and die, BUT if you don't want to risk killing yourself DON'T DO IT.
> We are going to try this in our next flight. Don't. You will likely invalidate your insurance and risk damaging the plane and yourselves.
> I don't want to die!!!!!! And if you do, everyone will say "Well, they took off outside the manufacturer's stated limitations. That was a stupid thing to do and was the cause of the accident. What a waste."
Of course, if your instructor is an experienced test pilot that would shed a slightly different light on it...up to you really...
Well... You might need to trim up briefly to assist in recovering the horrible dive resulting for the inevitible spin occurring after the first departure from perfectly controlled flight. Have you seen yet how quickly the ground appears to rush up at you while you're doing such irreponsible things?
Yes, new instructor, and your own decision making skill needs some review too!
I suspect that the instructor isn't really suggesting that the student fly with a CG out of limits, and that the point being made is probably being mistaken.
Perhaps the point of the instructor is to discuss download on the horizontal stab. That this was his or her intent, and that the lesson was either miscommunicated or misunderstood, makes more sense than as originally presented in this thread.
Assuming that when you refer to your instructor, you mean a genuine, certified flying instructor and not a bloke down the pub whom you bought a few pints and gave $100 for a few lessons, then I’d agree with SNS3Guppy’s assumption that you have either misheard or misinterpreted what you were told.
I suspect the point your flight instructor is trying to make is that with aft trim applied ( assuming a trim tab and not a spring bias system) you will have more control authority due to the now down position of the trim tab, albeit with a higher control load.
Trying it for real though I would leave well alone.
RTFQ. He is correct. You will have to trim FOR nose up (i.e. to compensate for a nose-up pitching moment). I suspect that he will demonstrate performance with aircraft loaded near the aft CofG limit, not exceeding it. Less stable in pitch, more responsive; shouldn't kill you.
you need forward trim as the stall is already more vicious at AFT CoG, so you need all the nose down authority you can get---perhaps he was speaking of a tail plane stall--which can occur due to ice accretion ---in which case the horizontal stabilizer is stall causing a severe [perhaps uncontrollable] pitch down moment
Up trim tab-down elevator, bang on mr Pugilistic Animus. but if you now push full forward you have full down elevator MINUS the little bit of extra effort that the DOWN trim tab could have given you.
As for all moving tails, well I don't think that's what we're talking about here,certainly not what i'm talking about anyway!
when I wrote stab meant H. stabilizer--Not stabilator sorry--
i.e Up trim tab ---down elevator--- up {tale}--- down nose
a PA-28-181 has a Stabilator --as does a few others..of that genre
Every CFI should demonstrate an 'Elavator trim stall' and with some willing students-- a stall a full aft CoG---you wouldn't dare trim nose up afterwards--the stall is easy and vicious--trust me been there done it---
PA
Last edited by Pugilistic Animus : 28th July 2008 at 16:25. Reason: change aileron for elevator--- Emboldened
Must confess I don't have a clue. With a CofG out the aft limit the stick force per "G" reduces to a very low limit and controllability becomes a real issue, as in PIO for example. In a word, you "die". Recommendation - don't try it.
Conventional light aircraft have the CG ahead of the wing centre of pressure and thus the tailplane produces a downforce to counter the nose down pitching moment of the wing. If you move the CG aft, the moment arm to the wing centre of pressure decreases, thus the need for downforce on the tailplane decreases, i.e. the stick needs to move forwards.
As the CG moves aft the static margin reduces, causing decreased static stability and making the aircraft more sensitive in pitch. Eventually, if the CG moves back far enough, the aircraft becomes statically unstable in pitch and thus much harder to control and more prone to stalling. The directional stability also decreases, making the aircraft more suceptible to dutch roll. If that's not bad enough, aircraft handling post-stall is worse for an aft CG, and the stall itself is often more violent. Other nasty things that can occur with a CG aft of limits are over-rotation on takeoff, leading to tail-stike or a stall, or PIO in pitch, particularly during the landing flare.
All in all, a bad situation. The aft CG limits is there for some very, very good reasons, and must not be exceeded. When a test crew is doing aft CG envelope expansion, it is approached VERY carefully, in an incremental fashion. The simple message is: DON'T DO IT. And if your instructor really said what you said he did, and he meant it, and you didn't misunderstand, you might want to get a different instructor....
an 'ELEVATOR trim stall' is an FAA concoction --- used when one wants to explain how easy it is to stall in terms of stick force with back trim [like configured for a landing]-oops
Pug, I'm not sure of what you are referring to here. The only "trim stall" that I'm familiar with is stabiliser trim motor stall when airspeed is so high that the trim motor is unable to counter the aerodynamic loads associated with an out of trim condition.
" We are all apprentices in a craft where no one ever becomes a master." - Ernest Hemingway
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--- down nose
--the stall is easy and vicious--trust me been there done it---
trim stall' is an FAA concoction --- used when one wants to explain how easy it is to stall in terms of stick force with back trim [like configured for a landing]-oops
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