Why is there a difference in positive and negative load factor in an airplane?

Question

I failed my CFI checkride today because of this, and I still can't figure it out. I told the guy that this is just how the airplane is built, and I'm not an engineer but it is always a good idea to remain at or below maneuvering speed for your particular weight so you end up stalling before you exceed your airplane's structural limitations. Well that obviously wasn't good enough for him. He wanted to know why the positive load factor of 3.8gs (which is 2.8 gs away from the normal gravity force of 1g) is more than the negative load factor of -1.52gs (which is 2.52 gs away from the normal gravity force of 1g). I also said that negative g's might pull the fuel away from the tanks and onto the top part of the wing since they are gravity fed, causing the engine to quit. Everything else on my oral was perfect, I can't believe he failed me for this. I'd never heard this question asked, and neither had my instructor. He also kept asking questions out of some naval aerodynamic book, and kept telling me I should have read it. Is he for real? I am not in the military and don't think I should be held responsible for that information. P.S. we are using a Cessna 172 RG.

Oh and he asked me a couple multi-engine questions on my single-engine CFI checkride. Is this normal?

Thanks to anyone who responds.

Answer 1

Unfortunately, you received what I used to call the 'examiner from hell'. There was a probability of 1/1000 of receiving the examiner from hell during one of your check rides. You rarely got one on more than one check ride though.



Although I can barely remember my CFI check ride, I did learn about the +ve and -ve load factors. However, I was asked a question which I had not learned about but I managed to answer it (about long stability and the profound effects it has on the airplane's pitch.)



The fact this examiner was using questions from some naval aerodynamic book is something out of the extraordinary. I have yet to even know any examiner who would use this approach during such an important examination. It even sounds odd to me. However, I haven't read this book so it may be about the primary principles of aerodynamics - unless the questions were navy related?



If the examiner was using a naval-related book to question me, after many weeks of preparing for this examination, I would be let down and perhaps question his motives for using those particular questions instantly.



Multi-engine related questions for a Cessna 172 RG? Was this examiner under the influence or something? If I was you, I would have refused to answer such questions which was not related to the single engine CFI specification. Now, I suggest that you discuss this matter with your instructor as soon as possible about taking suitable action. Not only have you prepared for this difficult examination, but this examiner has certainly pulled a fast one. You must not let this get away because it is certainly unfair.







Good luck when you retake and please don't let that examiner get away! You didn't deserve his awful examination style... no CFI pilot in training does!

Answer 2

Positive

Answer 3

1. Find a new examiner. This guy is a jerk.



2. The real answer to the ? is that airplanes designed with more + LF than - LF are lighter than if the + & - LF were the same. This is accomplished with no reduction in structural reliability, and so it gives the designer a chance to build in more performance by having the empty weight less. In turn, the reason for this is that + g loads are statistically much higher than - g loads, simply because airplanes seldom spend much time upside down. Even planes designed to the acrobatic category are allowed to have the design -LF = ½ of the + LF.



3. This ? has no place on any pilot rating oral. It's quite sufficient that the pilot know there IS a difference........

Answer 4

Well the book he's using is called "Aerodynamics for Naval Aviators" and it is basically considered to be the gold standard in terms of aerodynamics textbooks, so yes, you probably should read it. That said, on to your question. I also find it hard to believe that this one question resulted in an unsat on a checkride. (This guy's initials don't happen to be R.N. do they?) But whats done is done and I guess its about moving forward at this point.



I think the basic answer he would have been looking for comes from the certification limits of the aircraft. It should be enough to say (in this case) that the structural limit of -1.52 Gs is there simply because that is the structural limit. That is all the aircraft is required to, or even tested to, withstand. Beyond that the aircraft is not operating inside its normal, or utility envelope and you are violating a published limitation on the airframe. This is referred to as the Design Limit Load Factor, which is where permanent deformation begins to occur. (even though something called an Ultimate Limit Load Factor is also designed into the airframe which is actually a 1.5G safety margin over and above the Limit Load Factor resulting in a total G loading of -3.02 Gs before structural failure sets in)



If he wants to split hairs though, there are probably some reasons for it. He may want to hear something about the design limits on the horizontal stabilizer (which usually fails first in an uncontrolled spiral-the most common result of inadvertent flight into IMC) or he may just want to hear you mention the certification limits. In any case, I would say the "correct" answer is simply that the -1.52G limit exists simply because it is certificated that way.



There were probably many meeting by engineers and regulators over the years discussing this, and there might be an argument to be made that in a negative G scenario the horizontal stabilizer is doing the opposite job it is designed for and may be much more likely to fail upwards, but that still goes back my original argument that the design limit load factors exist because of the design, or the certification of the airplane. Aerobatic aircraft frequently are designed to withstand negative G loads far in excess of -1.52, so it is possible to do so, its just that normal and utility category aircraft are not required to demonstrate this capability, so the aircraft themselves are not designed to withstand G loading in excess of -1.52.

Answer 5

There is no 3.8g condition. it is 2.5g maximum (3.75g ULTIMATE is just 1.5 times 2.5g, but you don't fly the airplane like that). The cert condition for negative maneuvers is 1.0g. (1.5 is ultimate).



Limit loads are maximum loads expected in the fleet's lifetime. Positive maneuvers are part of flying- for example, a dive pullout (2.5g) or a steep coordinated turn (2.0g for a 60 degree bank) whereas for non aerobatic flight -1g is very hard to achieve and never a normal maneuver.



I would say you need to go to the FSDO and complain. The Navy's test standards are not part of a CFI checkride; neither are type design standards.

Answer 6

They are different because of the aircraft design (in other words, you were right).



Most aircraft are designed to take higher positive than negative loads because they're more likely to encounter high positive loads than high negative loads. Since the total strength of the airframe has a strong influence on aircraft weight, building strength that isn't required makes the aircraft unnecessarily heavy. If a load of x is twice as likely as positive load than it is as negative load, then building in more margin for the positive load makes sense, and avoids the extra weight penalty that you'd have if you built for the same strength for negative loads as well.



But fundamentally the answer is that the aircraft is designed that way. You can easily design aircraft in other ways, but nobody does (except maybe for aerobatic or other aircraft that might be equally likely to sustain both heavy positive and heavy negative loads).

Answer 7

I failed a ride like yours once (1978) and was informed later that he failed me because FAA examiners don't get paid extra for giving check rides and he was wearing a suit that day. He asked me for an in-depth explanation of Aspect Ratio. My instructor went ballistic also. Perhaps, in his favor, he believed I was there for an A&P instead of a CFI. Good luck.

Answer 8

Sound like the guy who gave me my oral a long time ago.





It has to do with the designed structural strength of the air frame. The positive G loads are the esieast to effect, increrase the payload you increase the 1 G load on the air frame.



This site has explination of G forces



http://www.flightlab.net/pdf/8_Maneuvering.pdf

Answer 9

Look at a Vg diagram. Being at Va is relevant for positive--not negative--load factors.