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hank jarrett · 18 June 2009, 06:23 AM

First, even though the V-n diagram looks like there will be damage at just under 4 g’s and 150 mph, what the diagram is REALLY telling you is the design was TESTED to that g load from ~115 mph to ~180 mph. I would read that to mean I am safe to pull 3.5 g’s at any of those speeds. Now for a caveat, (aren’t there always exceptions?) That assumes the plane has NO damage or degradation in any affected load path (now you know why those annual inspections are so important and why the A&Ps jump all over such minor damage). It also assumes that there is perfectly smooth air and the g’s are applied gently.

That brings us to your gust load question. Those lines are calculated using the equations from the FAR’s. We will pass on the math here because at this point you just need to understand the results and I don’t want this to turn into a math class.

From the Feds on V-n diagrams;
Gust load is computed from the FAR Part 25 equation (the one we won’t be looking at). The formula considers a vertical gust of specified speed and computes the resulting change in lift from the wing. The associated incremental load factor is then multiplied by a load alleviation factor that accounts primarily for the aircraft dynamics in a gust.

Nice and clear? Don’t worry about it. It confuses the engineers at first too.

What it is saying is that a vertical gust changes the effective angle of attack of the wing and has the same effect as pulling back on the stick and loading the plane with g’s. As you can see from the V-n diagram, this effectively reduces the number of G’s you can pull in gusty weather. Sound familiar? That was what you learned when you were under instruction. If it’s bumpy, slow down! Now you know why, and more importantly, HOW MUCH to slow down. Since it is kind of hard to get Mother Nature to oblige the test pilot with just the right gust for tests, this curve is calculated. Also remember, if the gust is HIGHER than 50fps the curve would be MORE restrictive. To know the total effect you would have to run new calculations for EVERY possible gust. A 50fps gust is a good working size and no one could remember (or carry copies of) all the gusts so 50fps is a good standard. Actually for certified planes below 20,000’ the calculations have to be done for 25, 50 and 66 fps.

What is being plotted on the diagram isn’t really failure modes. It is the maximum stress the design has been tested to. It is a lot more complex to predict which component will fail first. If engineers were perfect (and MAN ARE THEY IMPERFECT!) the V-n diagram would reflect exactly the maximum loads the plane could take, and when you put one gram too much stress on the plane, every rivet, bolt and piece of metal in the plane would fail simultaneously. The ultimate “One horse shay”. No wasted material anywhere and the absolute lightest design possible. V-n diagrams DO NOT show where the plane will fail. They show you where someone else has shown they WON’T fail

Hank

Anyone else who understands this stuff PLEASE look at it carefully. I can't exactly ask the guys at work to peer review what I am writing and remember my comment above, ENGINEERS ARE NOT PERFECT! If I make a mistake PLEASE let me know! This stuff can get people killed if it is misused or incorrect!

18 June 2009, 06:23 AM	#249 (permalink)
hank jarrett Scout Pilot Join Date: Mar 2005 Location: Virginia Beach VA Posts: 413	First, even though the V-n diagram looks like there will be damage at just under 4 g’s and 150 mph, what the diagram is REALLY telling you is the design was TESTED to that g load from ~115 mph to ~180 mph. I would read that to mean I am safe to pull 3.5 g’s at any of those speeds. Now for a caveat, (aren’t there always exceptions?) That assumes the plane has NO damage or degradation in any affected load path (now you know why those annual inspections are so important and why the A&Ps jump all over such minor damage). It also assumes that there is perfectly smooth air and the g’s are applied gently. That brings us to your gust load question. Those lines are calculated using the equations from the FAR’s. We will pass on the math here because at this point you just need to understand the results and I don’t want this to turn into a math class. From the Feds on V-n diagrams; Gust load is computed from the FAR Part 25 equation (the one we won’t be looking at). The formula considers a vertical gust of specified speed and computes the resulting change in lift from the wing. The associated incremental load factor is then multiplied by a load alleviation factor that accounts primarily for the aircraft dynamics in a gust. Nice and clear? Don’t worry about it. It confuses the engineers at first too. What it is saying is that a vertical gust changes the effective angle of attack of the wing and has the same effect as pulling back on the stick and loading the plane with g’s. As you can see from the V-n diagram, this effectively reduces the number of G’s you can pull in gusty weather. Sound familiar? That was what you learned when you were under instruction. If it’s bumpy, slow down! Now you know why, and more importantly, HOW MUCH to slow down. Since it is kind of hard to get Mother Nature to oblige the test pilot with just the right gust for tests, this curve is calculated. Also remember, if the gust is HIGHER than 50fps the curve would be MORE restrictive. To know the total effect you would have to run new calculations for EVERY possible gust. A 50fps gust is a good working size and no one could remember (or carry copies of) all the gusts so 50fps is a good standard. Actually for certified planes below 20,000’ the calculations have to be done for 25, 50 and 66 fps. What is being plotted on the diagram isn’t really failure modes. It is the maximum stress the design has been tested to. It is a lot more complex to predict which component will fail first. If engineers were perfect (and MAN ARE THEY IMPERFECT!) the V-n diagram would reflect exactly the maximum loads the plane could take, and when you put one gram too much stress on the plane, every rivet, bolt and piece of metal in the plane would fail simultaneously. The ultimate “One horse shay”. No wasted material anywhere and the absolute lightest design possible. V-n diagrams DO NOT show where the plane will fail. They show you where someone else has shown they WON’T fail Hank Anyone else who understands this stuff PLEASE look at it carefully. I can't exactly ask the guys at work to peer review what I am writing and remember my comment above, ENGINEERS ARE NOT PERFECT! If I make a mistake PLEASE let me know! This stuff can get people killed if it is misused or incorrect!