PLS Answer this SKILLED PILOTS!
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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
Is he crazy?
please explain if he is right
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?
Bart
Warning Toxic!
Disgusted of Tunbridge
Disgusted of Tunbridge
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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.
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.
Last edited by Rainboe; 11th Aug 2008 at 12:43.
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The trims effect on AoA
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
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
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A most unwanted practical example:
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.
Chr's
H/Snort
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.
Chr's
H/Snort
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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
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
