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4 Answers

Load Factor

Asked by: 920 views , , , ,
Aerodynamics, FAA Regulations, General Aviation

Hi! I have a couple questions...first I'm curious how easy it is to accidentally exceed load limits during maneuvers. I'm more familiar with positive Gs, and know what ~2G feels like from steep turns so I know exceeding the +3.8 would be unlikely in normal conditions. But having little experience with negative G I'm not sure what the -1.5G limit would feel like nor what circumstances would provoke that amount of force. What kind of force is involved in the "cell phone on the dash" trick where you do a slight parabola to simulate weightlessness? I want to make sure never to come near the -1.5G limit but without buying an accelerometer I don't have anything to go by...

My second question is during an annual inspection are structural components of the wing, empennage, etc. checked for fatigue or stress damage? I'm curious as my training aircraft is a ~40 year old PA-28 and imagine after that many years in service it must have some wear and tear. But looking through the recommended maintenance I don't see anything pertaining to checking wing spars etc. Surely checks for stress cracks, corrosion, and similar issues must be performed right? 

 

Thank you in advance for your wisdom!

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4 Answers



  1. Russ Roslewski on Nov 23, 2016

    For non-aerobatic flight, +3.8 G’s is a lot of force. You don’t achieve that accidentally by just turning too tight or pulling back a little too hard. You have to make an effort. If you are not used to aerobatic flight, you will likely stop due to being uncomfortable long before that. So, you’ll know.

    Similarly, -1.5 G’s is also a lot. The “cellphone floating on the dash” trick is accomplished at right about 0 G’s, and you feel weightless. To achieve -1.5 G’s requires determination, especially as since in a non-aerobatic airplane the engine may sputter and/or die. Further, it will be very uncomfortable. So, again, you’ll know.

    Yes, aircraft are checked for cracks in spars and such as a routine part of an annual inspection. Inspection panels are placed so as to allow this check on most aircraft. Granted, there are always some areas that the mechanic just can’t see.

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  2. Brian on Nov 24, 2016

    Hang upside down on a set of monkey bars. That’s -1g.

    The only time I see folks pull high g’s are when recovering from a spiral or spin after a botched stall. Pull out from a high speed, very steep (45 degrees or better), dive is likely the only time you’ll over g an aircraft. This also accounts for the majority of over g’ing an aerobatic bird as well.

    In an accidental steep dive you pull hard early and ease up as speed increases. Done perfectly you’re at the edge of stall till Va and backing off for structural safety after. In theory, of course. In reality a skilled pilot who knows their bird will feel it out and verify with the meter after.

    Your typical planes are not as rigorously inspected as you might think. The only part of the spar you can see well in a Piper is the portion running under the pilot seat. Most of the internal of the wing is simply inspected for corrosion. The majority of the inspection plates are for control cables, wiring, and fuel tank access.

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  3. 812fly on Nov 25, 2016

    Thank you both for your help…I’m probably thinking about this incorrectly but I’m still a little unclear on the G forces involved in the parabola…0G means no acceleration right? But in reality the pilot and the aircraft are both accelerating downwards at 9.8 m/s^2 and because of the zero relative acceleration we feel like there is no gravity. So since the plane is accelerating downwards at “G”, does that mean there is a 1G force being exerted on the plane? -1G?

    One other question: the POH for the Cherokee states “no inverted maneuvers approved” for the maximum negative load factor. It doesn’t even mention the -1.52G/-1.76G. Since the Cherokee is certified as normal/utility I assume it’s able to withstand at least some negative G forces. But why wouldn’t the POH list the certified load factors?

    Again, thanks for your help 🙂

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  4. Brian on Nov 26, 2016

    Think more in the relationship of lift to weight as that is how we calculate load factor. If you do a push over to 0 lift then the formula cancels out and you’re left with zero gravity. As for your Cherokee book it was published before the FARs started to regulate the limitations sections put in an aircrafts POH. In other words the information in there is about as useful as you writing your own POH.

    Even the POH today typically has only one section that is regulated and as such should be adhered to. The other sections are informative. Have a read through these two articles:

    http://www.avweb.com/news/pelican/186015-1.html
    http://www.avweb.com/news/pelican/186216-1.html

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