Where, not how, is VNE calculated. Does VNE go up with altitude?

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Faralon

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Example 1: Zenith Cruzer | VNE: 145mph.

Example 2: Viking Aircraft's Zenith "Super Cruzer": I just watched a video of it doing 180+ mph, you can see it on their YouTube page, filmed in November 2016.

Yes I know the Viking Cruzer has wings that are blended more aerodynamically to the fuselage/canopy than the factory Zenith as well as most of the control arms for the flaps have been moved inside the wing creating a slicker airframe.

However, the filming is taking place at 10,500ft. Would you be able to take the VNE of the factory Cruzer higher than listed (and still fly safely) at higher altitude?

I can't imagine VNE is calculated at ground level. These are, after all, AIRplanes. So how, or where, is the number calculated, and can you fly that aircraft (SAFELY) above the VNE at higher altitudes?

Not that I intend to do this, its just a curious question.
 

Marc Zeitlin

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Example 1: Zenith Cruzer | VNE: 145mph.

Example 2: Viking Aircraft's Zenith "Super Cruzer": I just watched a video of it doing 180+ mph, you can see it on their YouTube page, filmed in November 2016.

Yes I know the Viking Cruzer has wings that are blended more aerodynamically to the fuselage/canopy than the factory Zenith as well as most of the control arms for the flaps have been moved inside the wing creating a slicker airframe.

However, the filming is taking place at 10,500ft. Would you be able to take the VNE of the factory Cruzer higher than listed (and still fly safely) at higher altitude?

I can't imagine VNE is calculated at ground level. These are, after all, AIRplanes. So how, or where, is the number calculated, and can you fly that aircraft (SAFELY) above the VNE at higher altitudes?
Funny you should ask this - Steve Ells has a good article related to this issue in the current (Feb., 2017) issue of Sport Aviation, on page 26. Vd=Vne/0.9, and Vne =1.4 Vc (from part 23). But once you've chosen a Vne / Vd combination based on the cruise speed of the aircraft, you then have to flutter TEST to Vd - the calcs mean nothing without designing for the speed and then testing to it.

Most small planes list Vne in IAS or CAS. However, this is only a poor approximation of the truth, as flutter is a function of TAS - it's accurate enough at lower altitudes, but not so much as you get higher.

This article:

https://www.vansaircraft.com/pdf/hp_limts.pdf

has a good explanation of why Vne (in IAS/CAS) is LOWER at higher altitudes, since TAS is higher.

So unless this Viking "Super Cruzer" has been tested to at least 11% faster than the fastest speed they claim is Vne (which would be the new Vd), they're in uncharted territory, and at the higher altitudes, even moreso.
 

rv6ejguy

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Vne is often calculated as TAS so if you're using IAS up high as an indicator, you're way over Vne. Unless the Viking version was substantially beefed up and already tested there, the pilot was probably taking a big chance.

Was that 180mph IAS or TAS at 10,500 feet? 180 indicated is about 210 true at that altitude.
 

bmcj

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Marc and Ross gave you some good information. I'll try to add a little here with a little less technical bent:

When asking about Vne (at least for homebuilt aircraft), you have to look at which design/designer presents the value. Vne can be set based on many factors. It can be established as a flutter limit, structural limit, etc. Sometimes it can be chosen a bit arbitrarily by the designer hoping that they have picked a number that still has a large margin of safety (note: this is not the mark of a good designer). Additionally, sometimes they are set based on calculated values, and sometimes some will rightfully go a step further and include actual tests to confirm the calculation.
 

Battson

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Vne goes down at high altitudes (Vne airspeed reduces 3.47kts per 1000’ above 8000’) according to the designer of the Bearhawk.
 

bmcj

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How would you safely test VNE? Do you just pick an arbitrary number, then if it flies safely at a % speed faster than that, it's good, and if it breaks up and you die, you pick a lower number?
Realistically, if testing for a marketable value, you would take your calculated safe max speed, back it off to provide a desired margin of safety above what you want to publish, then test a chosen percentage beyond the published value but below the calculated safe max value.

For a homebuilt, your most likely limiter is going to be flutter. Flutter tendencies can be tested on a vibration table if you have access to one. Structural limitations can be set based on a combination of structure, speed, and design standard vertical air disturbances. There are some homebuilts that strive for lightness, and that can impose airspeed limitations. As an example, some use lightweight plexi for windows, but they can flex under higher speed air loads; many people will retrofit with a thicker plexi in order to expand their Vne limit.

Please note that I am trying to stay very 'untechnical' here, for your benefit and because my brain just isn't in a technically precise mood at the moment. There are others here that will probably chime in that can give you a very technical and correct explanation, much better than I can.
 

lr27

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Vne goes down at high altitudes (Vne airspeed reduces 3.47kts per 1000’ above 8000’) according to the designer of the Bearhawk.
I'm sure that's true for flutter limited Vne and indicated air speed. Not so sure for other conditions. It might be tricky, though, to prove that flutter isn't the first problem encountered, though I suppose you could if your aero engineer and the software are good enough. I can just see the testing now. A radio guided prototype towed behind a much larger, somewhat faster airplane. Or, using the skills of said aero engineer, and the inexpensive accelerometers we have these days, maybe some really minor vibrations might reveal that flutter speed is getting closer? I wonder how long it takes the amplitude to build up before it

I checked the table in the article, taken from a Pipistrel. It goes by true air speed. Out of curiosity, I looked at the situation if one were to take a mountain wave to 40,000 feet. At that altitude, 122 knots TAS is about 60.5 IAS. Stall is at 35.6 IAS. So, if you were to stall, better not gain more than 25 knots IAS (50 TAS), or less in an accelerated stall. Is recovery even possible that quickly in thin air?

I'm sure most of us have seen that video* of flutter testing a fiberglass sailplane. The wing is vibrating like crazy, but the test continues for a few seconds. I suppose on that sailplane, if you slowed down as soon as your fillings fell out, you'd be fine. ;-)


------------
*https://youtu.be/kQI3AWpTWhM
 

Marc Zeitlin

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How would you safely test VNE? Do you just pick an arbitrary number, then if it flies safely at a % speed faster than that, it's good, and if it breaks up and you die, you pick a lower number?
You test to the design Vd, by definition. We're in the process of flutter testing a Long-EZ I've been working on for a year now. The process is to start at some speed that is well below the published Vne (well, actually, to process starts when I take the elevators and ailerons off and ensure that they're balanced correctly, and then ensure that there's minimal play in the control system from the stick back to the control surface) and then work your way up to Vd. Stop before dying, if possible.

For a Long-EZ with a published Vne of 190 KCAS, we started at around 140 KCAS. Fly the airspeed and rap the stick hard in all four directions, ensuring that it's deadbeat (no oscillations or vibration). Then increase the CAS 5 kts, and do it again. Rinse and repeat until reaching the Vd of 211 KCAS. Obviously, this will require some serious diving and RPM's by the time you get to the higher CAS's - my test pilot was in an 11 degree, 4000 fpm descent at 2900 RPM on the 190 KCAS dive (fastest to date - we plan to finish to 211 KCAS tomorrow, and keeping to 2900 RPM will probably require a 20 degree dive).

Do this at the highest reasonable altitude at which a give CAS can be reached. Now, since we already know that TAS is what flutter is dependent on, not CAS, we want to ensure that we'll test at least at the highest TAS that the aircraft would reach in a descent from high altitude cruise. Since this airplane can cruise up into the high teens / low 20K ft range, should the pilot want to, we'll need to have a TAS restriction on the plane as well as the CAS restriction of 190 KCAS, and we'll set it to 90% of the highest TAS that we reach in the flutter testing, which so far has been done at 9K ft.

Spaceship Two was done in a similar manner, but dropping from 50K ft. We were never able to get above about M=0.85 in a gliding dive, and never at a high enough altitude/Mach condition to feel very good about it. And the plane was too big for stick raps to really give enough energy input to the system - we really should have been using some sort of control surface exciter motor, but those have their own problems associated with them that we didn't want to get into... It was an interesting set of tests.

I have to say, flutter testing my COZY MKIV was about the scariest thing I've ever done in an airplane...
 

Pops

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How easy to get in trouble. My Falconar F-12 had a VNE of 187mph. At 10K my TA at cruise was about 160/165. Any time you lowered the nose just a little the airspeed would have a large increase. After flying it for 5 years I knew that the airspeed had to be monitored at all times, BUT, this time I wasn't paying attention when starting to come down from altitude and wasn't thinking about the TAS. Noticed the IA just as it went thru 200 mph. Before I could pull the power off, I felt a shake in the stick, looked to my right and the TE of the wing tip was going up and down about 6" about one cycle a second. Pulling the power at the same time. The flutter stopped as soon as I pulled the power. After landing I checked the aileron cables, ( also checked the wings and ailerons, no damage). One turnbuckle had a single wrap on the safety wire and had turned a half turn for less tension on the cables. I reset the cable tension and used the double wrap safety wire on the turnbuckle. Double wrap of all turnbuckles for me from then on.
 

Tiger Tim

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Well, today I learned that Vne is in TAS. I would have thought it would be in CAS since that's really a measure of pressure on the airframe.

Is there a short answer for why flutter is caused by your TAS and not CAS?
 

lr27

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Marc,

Thanks for all that specific info.
-------------
Are homebuilders exploring testing by radio control yet? Telemetry is getting cheap these days, and the technology to control the airplane reasonably well automatically also exists. If I was (hypothetically) testing an ultralight, I'd probably just use manual control and some telemetry or on-board recording.
 

lr27

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Well, today I learned that Vne is in TAS. I would have thought it would be in CAS since that's really a measure of pressure on the airframe.

Is there a short answer for why flutter is caused by your TAS and not CAS?
Must be less damping but same speed. And I'd say Vne would only be in TAS if the limiting problem was flutter, or if the flutter speed wasn't much above the structural speed. Structural strength can also be a limit. I'm not sure just what the rules are, but certainly you could build an airplane that was more flutter resistant than strong. Have you handled recent composite skis? At least for the ones I grabbed, it's amazing how soft they are in bending and how stiff they are in torsion. I'm sure that's because of the layup used.
 

Marc Zeitlin

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Well, today I learned that Vne is in TAS. I would have thought it would be in CAS since that's really a measure of pressure on the airframe.

Is there a short answer for why flutter is caused by your TAS and not CAS?
I am by no means a flutter expert, although I played one in the SS2 control room a number of years ago, watching the telemetry data during testing. As if we could react fast enough to say "Knock it Off" and have them do it if we saw something on the screen in real time.

But my understanding from talking to the experts who WERE there is that it's very complicated, but as lr27 says, a large part of it is a damping thing - with far less air up high, there's less energy being taken out of the vibrating system by pushing against the air, but the actual air velocity (TAS) which is the driving factor in creating the vibration frequency, is still high. When the driving frequency and the structural vibration frequency get close to one another, AND the damping is low, you can get flutter.

He says, as if he really knows anything about this.
 

bmcj

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Is there a short answer for why flutter is caused by your TAS and not CAS?
All structures have their own natural harmonic frequencies. If you expose them to those frequencies, flutter begins and grows. When you blow a slow wind across a reed, it just flows around the reed unnoticed. If you blow fast enough, it will start the reed vibrating. Air moving across an aircraft surface fast enough will excite the surface's natural frequency, and that excitation amplifies quickly. What matters in this process is the speed the molecules flow past the surface, not the number of molecules.
 

Marc Zeitlin

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Must be less damping but same speed. And I'd say Vne would only be in TAS if the limiting problem was flutter, or if the flutter speed wasn't much above the structural speed. Structural strength can also be a limit. I'm not sure just what the rules are, but certainly you could build an airplane that was more flutter resistant than strong...
This is true, depending upon what gust limit you want to use for your airplane.

The COZY MKIV V/N diagrams (which I developed, trying to use the published information for the airplane as the basis, since there weren't any at the time and it was a good exercise) can be seen at:

http://cozybuilders.org/performance/

You can see that in some cases, Vne would have to be set lower than the published # when up high and at light weights - higher wing loading obviously decreases the effect of gusts. With 25 fps gusts Vne is a flutter limit (to the extent that THAT'S what folks have tested to, not that anyone actually knows where flutter will occur in a properly built COZY MKIV). But with 50 fps gusts, Vne might have to be set lower.

The appropriate gust limits can be found in CFR 14 Part 23.333. For Vd limits, 25 fps is the one to use, and in all cases, the aircraft is well within G limits for a 25 fps gust at Vd. Yay.
 

BBerson

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Maybe some temporary control surface limiters could be fitted for the test. Limit travel to say 7° and see if flutter can be tickled.
Something like the 1x4 boards used for gusts locks in high winds at tiedowns. (common in Alaska)
Might cause a problem, so I won't recommend it, just an idea.
 

BJC

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Well, today I learned that Vne is in TAS. I would have thought it would be in CAS since that's really a measure of pressure on the airframe.

Is there a short answer for why flutter is caused by your TAS and not CAS?
The short answer is that it is not.


BJC
 
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