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Exceeding Vno, where is the danger?

SMO

Well Known Member
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Curious as to how others approach exceeding Vno, into the yellow arc on the ASI. The normal cautions are to only be in that regime when in smooth air, but what defines smooth air?

Can I use my g-meter to measure the cutoff between smooth and non-smooth air? For example, if I experience turbulence that shows up on my g meter, say .5 to 1.5, should I be slowing down to Vno? What about if it displays 0.0 - 2.0? Or greater?

Can I safely pull 3 g's in a loop when exceeding Vno if I am in "smooth" air? Or perform a roll where positive g's are maintained and do not significantly exceed 1.0?

Can someone provide an understanding of the aeronautical engineering process that goes into determining Vno?
 
Google "V-g diagram".
Vno is set where the load limit is reached in a instantaneous 30 ft/sec downdraft. (Old definition. I believe there is a revised standard which applies the force in a not-instantaneous fashion, but gives similar results).
 
Otherwise known as a "design gust."

A design gust is significantly stronger than you think. It's enough to exceed a -3 acceleration, which is probably the kind of turbulence that's serious enough to just keep you on the ground, rather than be in the air wondering what speed you should be flying at.

Also note that the yellow arc has nothing to do with Va, the speed at which full control deflection exceeds the aircraft's maximum load factor. That seems to be a point of confusion among some pilots I talk to, so it's worthy of note here.

- mark
 
Google "V-g diagram".
Vno is set where the load limit is reached in a instantaneous 30 ft/sec downdraft. (Old definition. I believe there is a revised standard which applies the force in a not-instantaneous fashion, but gives similar results).

Ok, so is there a way to measure this? How would this feel in the cockpit?

FYI, my Rocket has a Vno of 160 knots. My normal cruise at 23 squared, 3000 ft ASL, LOP, is >170 knots. This seems to be the airplanes happy place. I sometimes get into a bit of light chop, especially when crossing converging valleys, and get to wondering - when should I pull the power back? At the onset of any bumps at all?
 
Curious as to how others approach exceeding Vno, into the yellow arc on the ASI. The normal cautions are to only be in that regime when in smooth air, but what defines smooth air?

Can I use my g-meter to measure the cutoff between smooth and non-smooth air? For example, if I experience turbulence that shows up on my g meter, say .5 to 1.5, should I be slowing down to Vno? What about if it displays 0.0 - 2.0? Or greater?

Can I safely pull 3 g's in a loop when exceeding Vno if I am in "smooth" air? Or perform a roll where positive g's are maintained and do not significantly exceed 1.0?

Can someone provide an understanding of the aeronautical engineering process that goes into determining Vno?

You can do anything you want beyond Vno, as long as you don't hit that 30ft/sec gust. Well, you say, how do I avoid that? Well, you can't. Looking at it from another way, the designers have to draw a line somewhere. Somehow, long ago, based on some sample of atmospheric data, they came up with 30 fps as a reasonable gust level to design to, likely thinking that your chances if getting a higher gust level are small. So that is what the chose, and they chose Vno as the speed below which that gust won't bust the plane. It's just like Normal Category airplanes are designed to 3.8g and utility 4.4g (or whatever it is). What is to stop either class of airplane from exceeding these limits? Nothing except the pilot's interest in staying alive. Tnese are arbitrary limits that somebody, or more likeLy a group of people, determined to be reasonable, and it seems to have worked pretty well.

If you want to fly faster than Vno, without your airplane coming apart, you need to be aware that the strength of gust you can cope with is now reduced. Given that you can't tell when and how strong the next gust will be, you had better be damned careful when you encouter any turbulence at speeds higher than Vno.
 
Given that you can't tell when and how strong the next gust will be, you had better be damned careful when you encounter ANY turbulence at speeds higher than Vno.

I bolded/capped the critical piece of your response that starts to answer my primary question - if I read you correctly, for your risk tolerance you would get below Vno at the first sign of turbulence. Or perhaps you would never fly above Vno? Or is this the engineer in you responding versus the pilot :)?

I sincerely value your input.
 
Turbulence definitions: NOAA

Scott pretty much hit it on the head. The only thing I would add regards the significance of that 30 FPS gust.

NOAA defines light turbulence as gusts 5-19 FPS and moderate turbulence as gusts of 20-35 FPS. So, that 30 FPS gust is close to the upper end of moderate turbulence.

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 :D

NOAA Reference
 
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NOAA defines light turbulence as gusts 5-19 FPS and moderate turbulence as gusts of 20-35 FPS. So, that 30 FPS gust is close to the upper end of moderate turbulence.

That helps a lot also. Most of the turbulence I have encountered would be categorized as light.

Would be really interesting to know if there is any correlation between gust levels and what a person is likely to register on a g-meter, assuming an RV-like aircraft. Light chop always seems to show up on my g-meter as an extra +- ~0.5 g.
 
Great discussion here! I have had some of the same questions and the contributions are helpful.

:)
 
Now that this door is opened lets explore a little . . .

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?
 
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 :D

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.
 
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.
 
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?
 
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
 
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

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?
 
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.
 
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...

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.......
 
....maybe utilized a stainless brace for the spar attach?....

Be thoughtful and do some homework with stainless steel structural parts. Much of the stainless world involves annealed 300 series stainless. While it has an ultimate strength somewhat greater than 2024, it has a yield strength that's somewhat weaker than the aluminum.

Still, not all vendors use the annealed 300 series stainless. Some use better stuff.... so please do check.

Also, and this could potentially be an issue, it's not merely the strength you need to consider. Stainless (or almost any steel, for that matter) has a modulus of elasticity that's almost three times greater than aluminum. How does that affect you? It means that if a stainless part and an aluminum one are the same thickness and are sharing a load, the steel one will carry roughly 3/4 of the total load. Yes, there are certain assumptions that go into this estimate, but it's still generally valid. If the structure is designed for that, fine. But if not....

Dave
 
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