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  #11  
Old 07-26-2017, 11:08 PM
BobTurner BobTurner is offline
 
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Quote:
Originally Posted by kaa View Post
The current requirement for Vno is to withstand 50 fps gusts (https://www.law.cornell.edu/cfr/text/14/23.333), which according to the NOAA document linked above, corresponds to extreme turbulence.
Yes, this is the new standard. The old one was an instantaneous 30 ft/sec. The new one is 50 ft/sec but applied, not instantaneously, but ramped up from zero to 50 ft/sec over a short but finite time.
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  #12  
Old 07-27-2017, 05:49 AM
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Great discussion here! I have had some of the same questions and the contributions are helpful.

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  #13  
Old 07-27-2017, 05:58 AM
BillL BillL is online now
 
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Default Now that this door is opened lets explore a little . . .

Quote:
Originally Posted by BobTurner View Post
Yes, this is the new standard. The old one was an instantaneous 30 ft/sec. The new one is 50 ft/sec but applied, not instantaneously, but ramped up from zero to 50 ft/sec over a short but finite time.
So a youtube video was found about how to create a V-G (V-N) diagram for an RV7 using factory numbers. 167 kts for top of the green. Assuming possible structural damage zone above 6 g's and failure above 9 g's, how does one plot the 50 FPS gust on the V-G chart?

AND . . . doesn't the gust affect the wing and HS differently? Will a vertical down gust will unload the wing, but add to the down load on the HS?

Any good technical reference book covering this?
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  #14  
Old 07-27-2017, 06:23 AM
AusFlyingBear AusFlyingBear is offline
 
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Quote:
Originally Posted by Reformed SeaSnake View Post
Everybody has a different tolerance for risk, but it would not be too smart to fly into an area of forecast moderate turbulence above Vno
I think the word forecast here is critical. I would not plan above Vno if turbulence was forecast. In addition, if you experience unforecast moderate or severe turbulence then slow to below Vno as soon as possible.
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  #15  
Old 07-27-2017, 07:02 AM
David-aviator David-aviator is offline
 
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This discussion is about philosophy of flying on edge or outside the envelope. What's the point? It is not conducive to longevity of airplane or pilot. If airplane does not meet needs, get one that does.

...age 78 and still enjoying it, albeit well within the envelope...some of same vintage are not.

Not to be critical but I hate crashes, there are too many of them. It's bad PR.
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  #16  
Old 07-27-2017, 09:09 AM
BillL BillL is online now
 
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Quote:
Originally Posted by David-aviator View Post
This discussion is about philosophy of flying on edge or outside the envelope. What's the point? It is not conducive to longevity of airplane or pilot. If airplane does not meet needs, get one that does.

...age 78 and still enjoying it, albeit well within the envelope...some of same vintage are not.

Not to be critical but I hate crashes, there are too many of them. It's bad PR.
The point is David (for me anyway), that an RV7 had an inflight structural failure in Hurricane Utah. The third of it's type. There was no definitive cause. A 17000 hr pilot knowing not to yank on the stick, yet something happened. No indication he was over speed, no definitive data on turbulence, seemingly a pleasure flight with his partner. So, we dummies want to know just what we are getting into. Not how to push the limit, but where it is to avoid it. No one is going to fly around at 142 MPH just in case it gets bumpy, so how do I measure the risk factor? Ignorance is not bliss in a 7. Does that help?
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Lord Kelvin:
“I often say that when you can measure what you are speaking about,
and express it in numbers, you know something about it; but when you
cannot measure it, when you cannot express it in numbers, your knowledge
is of a meager and unsatisfactory kind.”
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  #17  
Old 07-27-2017, 10:54 AM
spatsch spatsch is offline
 
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Quote:
Originally Posted by BobTurner View Post
Yes, this is the new standard. The old one was an instantaneous 30 ft/sec. The new one is 50 ft/sec but applied, not instantaneously, but ramped up from zero to 50 ft/sec over a short but finite time.
I think actually the more interesting number is:

Positive and negative gusts of 25 f.p.s. at VD must be considered at altitudes between sea level and 20,000 feet. The gust velocity may be reduced linearly from 25 f.p.s. at 20,000 feet to 12.5 f.p.s. at 50,000 feet.


That means you can go to VNE in 25.f.p.s. gust symmetrically applied.

Vno is a rather random number. Why draw the line at 50ft/sec not 45ft/sec or 55 ft/sec. E.g. commuter planes need to sustain 66ft/sec . So if you really want to play it safe maybe you should use that assumption to draw your Vno line. Somebody making a risk/reward judgment for me I guess.

Fact is as gusts go up you need to fly slower. So even if you are below Vno you are not safe you are just safer... .

I recall flying gliders in mountain waves and I am not sure I would want to hit a real rotor even close to Vno.

Oliver
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  #18  
Old 07-27-2017, 11:56 AM
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grubbat grubbat is offline
 
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Default Where is the demon

I'm with Bill on this. Understanding more about the design, its weak points, and what a builder can do to add a little margin isn't too much to ask. If the tails are the weak point and its because they are coming unzipped, I want to know. Of course we don't know why the tails are coming off except in the few cases where the pilot exceeded VNE/or G loads.

A typical push back to attempting to increasing the perceived margin is that it will move the risk to another area of the design. My perspective is that if I had that knowledge, then l will make a decision as builder. Who knows, maybe I want to move more risk from the tail to the wings or whatever. At least I have more information to make a decision. Heck, half the fun in experimental aviation is increasing the knowledge level.

The scenario that keeps me up at night is the one where the RV pilot is humming along minding his own business at 180mph true in calm winds and a gust of wind removes the tail. (Of course this never happens except for the few cases that Bill referenced...) As I descend uncontrolled, do I wonder if I should have thrown out that RV-7 tail and gone with the RV-8? Or maybe utilized a stainless brace for the spar attach? I imagine at that point I would have given my life for just a little bit more margin...

Good discussion. Thanks to the OP.

CJ

Quote:
Originally Posted by BillL View Post
The point is David (for me anyway), that an RV7 had an inflight structural failure in Hurricane Utah. The third of it's type. There was no definitive cause. A 17000 hr pilot knowing not to yank on the stick, yet something happened. No indication he was over speed, no definitive data on turbulence, seemingly a pleasure flight with his partner. So, we dummies want to know just what we are getting into. Not how to push the limit, but where it is to avoid it. No one is going to fly around at 142 MPH just in case it gets bumpy, so how do I measure the risk factor? Ignorance is not bliss in a 7. Does that help?
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  #19  
Old 07-27-2017, 08:36 PM
David-aviator David-aviator is offline
 
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Quote:
Originally Posted by grubbat View Post
I'm with Bill on this. Understanding more about the design, its weak points, and what a builder can do to add a little margin isn't too much to ask. If the tails are the weak point and its because they are coming unzipped, I want to know. Of course we don't know why the tails are coming off except in the few cases where the pilot exceeded VNE/or G loads.

A typical push back to attempting to increasing the perceived margin is that it will move the risk to another area of the design. My perspective is that if I had that knowledge, then l will make a decision as builder. Who knows, maybe I want to move more risk from the tail to the wings or whatever. At least I have more information to make a decision. Heck, half the fun in experimental aviation is increasing the knowledge level.

The scenario that keeps me up at night is the one where the RV pilot is humming along minding his own business at 180mph true in calm winds and a gust of wind removes the tail. (Of course this never happens except for the few cases that Bill referenced...) As I descend uncontrolled, do I wonder if I should have thrown out that RV-7 tail and gone with the RV-8? Or maybe utilized a stainless brace for the spar attach? I imagine at that point I would have given my life for just a little bit more margin...

Good discussion. Thanks to the OP.

CJ
Craig,

The discussion is interesting but only to a point because it is 98% speculation.
Vans has flight tested all models and set the limits accordingly. To speculate that this part or that part is a weak link is just that - speculation.

How does one move forward with no definitive data or information. Back to the drawing board? Start over with a clean sheet?

The end result will be the same, design to a certain standard, flight testing with reasonable margins, same thing that exists now. The process is to test to some limit without failure and back off to provide a margin of safety. No one tests to destruction.

OK so readers want to know what all goes into designing and flight testing and setting the limits with regard to gust loads and airframe loads. Good questions. Design engineers can provide answers but they won't be simple.

Most of us are here because we love to fly, not design airplanes. We rely on the system to produce reasonably safe flying machines. I think that is a reality with Vans airplanes. They are safe if flown within set limits and not over stressed like what happened with the RV-8 demo flight.

On any day, any pilot can induce an inflight break up, it is not hard to do. But it won't happen if the pilot uses common sense. That's part of being an aviator and not die doing it.
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  #20  
Old 07-27-2017, 09:11 PM
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grubbat grubbat is offline
 
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Agreed.
During my previous life, my primary job was making structural repairs as per engineering orders from aeronautical engineers. Often times there would be a back and forth as to what they wanted done and what could actually be done within the confines of both the existing structure and the repair that they wanted to do. I learned a lot from those years and have lots of respect for the designer and their designs. Many of the questions we seek will probably remain speculation unless someone or some group wanted to fund some sort of analysis on the structure in question. Years ago there would probably have been several who would have volunteered in the experimential realm. Not sure if the experimential world today would be receptive to this endeavor. Seems like the prevailing opinion is don't ask questions, don't test the design, and be happy you have what you got......... so to those folks, I'm happy with what I got, I'm not pushing the limits, and I won't make any changes that may increases the margins. But I will think about it and hopefully that is still acceptable...

Quote:
Originally Posted by David-aviator View Post
Craig,

The discussion is interesting but only to a point because it is 98% speculation.
Vans has flight tested all models and set the limits accordingly. To speculate that this part or that part is a weak link is just that - speculation.......
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