Control surface unporting

[Dave Morgan 68]

Hi.I know there are a lot of real pilots here and hope one or more of you can help. I've just been spending evenings rereading Fate is the Hunter (again) and noticed an expression that slipped by me on previous occasions.In the last chapter he narrowly escapes (another) early end when flying a DC-4 I think, without one elevator hinge-pin. His explanation of the near loss is a little unclear and given as part of a conversation with, presumably, Benny Howard:" 'Unporting is the balance destruction of the elevators by aerodynamic force. I won't confuse you with the theory, but if enough separation between the fixed and the balance portion of your elevators occurs, your aeroplane will go into a vertical dive or even beyond the vertical, and no two men in the world are strong enough to bring it out...' "Howard goes on to describe how airspeed will alter centre of gravity, which strikes me as being somewhat mistaken and possibly confused with pitch.I still don't know what unporting of a control surface is, and a quick search turns up only a few equally uninformed arguments and references to losing fuel. Can anyone shine any light for me?Thanks.D

[scottb613 1,186]

Hi D, When speaking of "unporting" today - as you stated - we are normally referring to unporting the fuel tank pickups - so many low wing GA aircraft have finite amount of time defined in the POH for conducting a slip... As it applies to the context you used above - I would suspect that the control surface is put in a position where it is not getting a normal flow of air over it - and - therfore loses it's ability to act in a normal manner.. I believe T tails such as the DC-9/MD-80 line had this issue - hence the addition of a stick pusher to the cockpit - to prevent the pilot from getting into a stall where the control surface has no/limited effect... Hmm - I'm still not sure that is what he is referring to... Accelerated stall - perhaps ???Regards,Scott

[ZachLW 37]

<p>With changing airspeed center of lift changes,  not CG.  So you are correct there.In context, it would seem "unporting" would be the control surfaces "locking up" due to aerodynamic force and the failure of a particular part.  Perhaps if a hinge broke and the balance tabs were forced into the slip stream more then they're designed for...  It's sort of hard to put into words what I'm thinking.</p>

[Dave Morgan 68]

Ah... a little light may just have come on. I think you both may have parts of a good explanation there.I wasn't aware of balance tabs so I went looking. Could it be that the hinge to which the author, Gann, refers is on a balance tab? I suppose if that broke then the balance tab could end up acting as a sort of semi-fixed trim tab. Further, I see there are combination trim + balance tabs in existance.A passage or so later Howard explains to Gann how he may have cheated death by failing to slow his plane when it started to vibrate as he assumed it was just some familiar roughness in one engine. In that case he wouldn't have had to adjust the trim so possibly avoided making the situation worse. I'll have to sit back and think for a while about airspeed, centre of lift and pitch trim.Does that make sense? And do you know what kind of tabs are on the DC-4 elevators?Cheers,D

[ZachLW 37]

The "aerodynamic unporting" issue was apparently a phenomenon experienced exclusively in the DC-4. Interesting.