PLS Answer this SKILLED PILOTS!

[Try Hard 4.0]

My instructor says that if you have your CG exceeding aft limits, you have to trim for nose up.

Is he crazy?
please explain if he is right

Please answer this so that I would know.

We are going to try this in our next flight.
I don't want to die!!!!!!

[robdesbois]

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...

[ZFWT]

Keerrrraaaazzzzzzy

[The African Dude]

Actually, he's wrong. With the CoG outside aft limits the aircraft would pitch up, so to overcome the stick forces you'd need to trim nose-down.

Fire him and get a new instructor - one that doesn't teach you to fly with you mass and balance out of limits. :ugh:

[Pilot DAR]

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!

[SNS3Guppy]

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.

[Tightflester]

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.

[DB6]

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.

[The African Dude]

DB6 - wouldn't that entail trimming AGAINST the nose-up tendency? :confused:

Either way - Try Hard 4.0 seems to have disappeared, leaving us to debate the meaning of his question... and ponder his fate :ooh:

[Pugilistic Animus]

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:confused:

[Pugilistic Animus]

Up trim tab ---down elevator--- up stab--- down nose:confused:


PA

[Pugilistic Animus]

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

[greuzi]

Been over a week since 4.0's original post.

Is there a link for searching Google Maps yet?...and to rescue the thread....has anybody any thoughts on that disappearance too?

[Brian Abraham]

you have to trim for nose up
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.
aileron trim stall
Excuse me!!!.

[WeekendFlyer]

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. :uhoh: :eek:

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....:rolleyes:

[Pugilistic Animus]

Thanks Brian_Abraham


an 'ELEVATOR :O 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


Edited above


PA

[Brian Abraham]

'ELEVATOR trim stall' is an FAA concoction
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

[Pugilistic Animus]

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.

Yes, correct---but I'm simply referring to small SE FAR part 23 aircraft under 6000 LBS GW---You set in a lot/full [check AFM/POH!!] or aft trim while maintaining Vapp/slow flight and attempt a 'go around'-see what occurs;)

See Airplane Flying Handbook FAA 8083-3A--for greater detail for conducting the exercise with {Stoodants}:}

[Brian Abraham]

Thanks Pug, I'm on the same page now. :ok:

[Capot]

Well, I'm fairly experienced, but not an expert, so ignore my 2d/2c worth if you wish...

Once you fly the aeroplane outside its design limitations all bets are off.

Discussing how best to control it if you fly with the CG outside limits makes as much sense as a discussion on how to control it with a wing off, if you'll permit a slight exaggeration to make the point.

If an out of CG limits situation is created during a flight, that's because you have made a gross and silly error somewhere along the way, such as allowing a load to shift by not tying it down properly, throwing out some passengers (with or without umbrellas) without working out the consequences on your W&B, or simply not working out where the CG will be before you start.

We could always have a discussion about the best way to control the aircraft it you decide to go without any fuel aboard and the engine quits.

To quote my CFI "If you want to kill yourself, fly with the CG out of limits. WORK IT OUT."

[bArt2]

My instructor says that if you have your CG exceeding aft limits, you have to trim for nose up.

Is he crazy?
please explain if he is right


Why don't you ask him to explain exactly what he means.

Anyway, there are so many things you can learn while flying inside the aircraft limitations, so why bother learning how to do things outside of the envelope? :rolleyes:

Bart

[poss]

It has been a few weeks, I'm interested to know whether the student was mistaken or the instructor was actually crazy.

[Rainboe]

What's the confusion? He meant if you are at aft limit CG, or inadvertently out of aft limit CG, trim for nose up, not trim nose up. Trim for nose up, ie trim nose down. He is not implying that at aft limit CG, you should apply nose up trim, God forbid!

A case of query meaning at the time and understand what is being said, rather than note down and run to Pprune with the wrong impression! A little thought brings the realisation we are dealing with a language problem here where simple misunderstandings occur.

[Bernie_nor]

Hi!

Like Rainboe, Bart and others have said: Ask the instructor what he means and whats the purpose of the exercise.

I've only flown gliders so my experience with the C172 is shall we say slightly limited. :)

I've done quite a bit of spinn training i various gliders. One common thing with all of them that has a trim tab (like the C172) is that they are more willing to spinn if you trim them nose heavy instead of tail heavy. The reason for this is quite simple. While trimming the plane nose heavy, the elevator is more effective pushing the tail down. This gives us a greater AoA with the stick at its rear limit then if the plane was trimmed tail heavy.

When flying gliders a situation with the CoG behind the rear limit can occure in two cases:
1. The ground crew forgot to remove the tail dolly (removeable tail wheel). And you forgot to check it before takeoff.
2. Flying with waterballast in bot the wings and the tail in to cold weather. When dropping the water the water in the tail will freeze up before the water in the wings.

The solution in the first case is to land as quickly as possible, and keep your speed well above the stall speed. You can easily notice during takeoff that you have the dolly on.

The other cas is more subtile. It's nearly impossible to notice. But if you suspect that the water in the tail have frozen (and you have let go of the water in the wings). Then again keep the IAS well above stall speed at all times.


Best regards
Bernie

[hoggsnortrupert]

A/C was a BN 2:

1994, a chap that couldn't be told, loaded his A/c with sea slugs, at an away port, he loaded all the freight in the rear.

He and the owner of the freight then jumped into the front seats and blasted off, witnesses report the A/C over rotating on take off, but became stable and flew on.

On landing, the sod on short final put flap down, the A/C pitched up dramatically , stalled, crashed, burnt, no one survived, (the sea food was over cooked).

The chap had to know from the T/O that he had a C of G problem, yet he did what he did.

A short while later I had to lower myeslf from the "Twotter" and fly a F-----g "grinder" into a smallish strip:

The A/C had just come off check, I did a cockpit check of flight controls and trim movement, and positioned the trim index to the T/o position.

On approach to landing the aircraft pitched up rather violently when I lowered flap, and I could not hold it with full forward elevator, instinctively i raised the flap, and resumed elevator control, by applying some extra power and speed "if you can for a grinder", the end result was a mal aligned trim wheel & cable, an engineering issue.

The landing was uneventful apart from some laundry issues.

So the trick is to:

Place the trim wheel in the neutral position, count the forward rotations and back, then look at the bloody thing to see it is where it should be, then again fully aft and back, again counting.

The point I am making is don't go there, don't even contemplate it.

The books have the weight & Balance graphs, use them, and when I happen to fly a Prop powered machine, I roll the trim fully forward, get out and look at the elevator trim, get back in roll it fully aft, get out and look at the elevator trim, then center the trim wheel and look at the elevator trim.

Yes the F/O's think I am weird and you can not explain to them why, they are just not interested.

But I still ask them to go out stand at the rear and signal to me when the trim tabs etc are fully up, neutral, and fully down.

But as horrible as it sounds, having to pry the deceaseds burnt hands off the power levers where all the plastic had melted into the flesh & bone!!!, I just wish these F/O's that think it such a joke, had to endure what I had.:mad:

Chr's
H/Snort

[HarleyD]

This has been a very interesting thread to monitor. FT threads lack the vitriol found in many other fora, especially those of my compatriots it would seem.

I agree that the real point that the nstructor was attempting to state should ascertained by the neophyte and passed on to this place if possible, however if not then there are a cuople of comments that I would like to make.

If the point being made was that the additional camber effect of a down turned trim tab would provide additional lift to assist with the raising of the tail when in an aft CofG, then this is not the way to convey that information, because ,whilst true, it neglects the differing configurations of trimming methods (trimming stabilizer etc) in which this procedure will most definately result in an adverse effect.

If the instructor intended that the trim be used correctly - ie trim for nose down when configured aft CofG then this would seem to be stating the bleeding obvious and a little redundant.

If however the instructor was making the point that "if you find yourself in a situation where for some uncontrollable/unpredictable reason the Cof G has become (extreme) aft, then this might help..." he should have made that point quite specifically and added that this will only assist on certain types. - this would be an illustration of transfer of 'tribal' knowledge.

For part 23 the trimmablity of the vehicle is required to be demonstrated for a range of CofG, power/flap/gear combinations, and this along with the demonstrations of longitudinal control (not stability) and to a lesser extent the control forces for control during landing determine several major aspects of the CofG envelope. To provide any encouragement to a student they can operate outside the envelope by using 'tricks' is completely enethical and potentially lethal. Ther are some parts of the compliant envelope where it is acceptable to apply temporary two handed control wheel forces of 75 lb, single handed of 50 lbs and sustained forces of 10 lbs, so adding additional forces by adverse setting of a moveable tab and and aft CofG outside demonstrated limits, may well exceed the pilot's available strength.

I am not aware of the term 'trim stall' relating to any particular part 23 demonstration for the purpose of compliance, but clearly see the trim motor overload case as bing a suitable definition.

I think that the information contined in the above posts would indicate that a more appropriate comment from the instructor would have related to the careful and minimal application of flap for landing when an inadvertant aft CofG had been identified. application of extreme adverse positional trim seems to be poor advise. If hawever the advice was to set the trim in this position PRIOR to flight when an Aft CofG case has been identyified PRIOR to flight, then this is just utter stupidity.

For the FT I have been involved in thses cases are approached very carefully with the CofG moved to the aft position whilst in flight at a safe speed and height and with the additional added precautions of emergency egress available. accurate data logging and control force application is vital to establish the safe operating limits as well as repeatable limits for the demonstration fo compliance. You don't just load the 172 to the gunwhales, wedge the aft locker full of suitcases and tie down kits then squeeze the doors shut, applt full nose up trim start engines and attemt to depart on a scenic tour as the results will not be pretty.

HD