Vne

[jp]

I know that Vne can vary with altitude but does it also vary with aircraft weight?

[Never Again 2]

At 18:49 09 November 2019, jp wrote:
>I know that Vne can vary with altitude but does it also vary with
aircraft
>weight?
>
Yes

[Never Again 2]

At 18:49 09 November 2019, jp wrote:
>I know that Vne can vary with altitude but does it also vary with
aircraft
>weight?
>
Yes

[Charlie Quebec]

Onlynif over max weight and with an engineering order. Within the certified weight range, it stays the same.

[Eric Greenwell[_4_]]

Charlie Quebec wrote on 11/9/2019 12:53 PM:
> Onlynif over max weight and with an engineering order. Within the certified weight range, it stays the same.
>
It may depend on the glider, particularly gliders that carry ballast in the wings.
My guess is pilot weight would have such a small effect, it would not mentioned in
the operating handbook.

--
Eric Greenwell - Washington State, USA (change ".netto" to ".us" to email me)
- "A Guide to Self-Launching Sailplane Operation"
https://sites.google.com/site/motorgliders/publications/download-the-guide-1

[Charles Longley]

I’ve never seen it vary by weight. Check your POH of course

[Eric Greenwell[_4_]]

Charles Longley wrote on 11/9/2019 6:22 PM:
> I’ve never seen it vary by weight. Check your POH of course
>
Now I'm wondering if it's a regulatory requirement that the Vne be set that works
for all allowed weights.

--
Eric Greenwell - Washington State, USA (change ".netto" to ".us" to email me)
- "A Guide to Self-Launching Sailplane Operation"
https://sites.google.com/site/motorgliders/publications/download-the-guide-1

[Echo]

Ehhhhhhh no. In our very sub supersonic world Vne is based on how fast the molecules actually go over the wing, which is the contributing factor to flutter. That's Vne, more specifically, it's given as a True Air Speed because that's the actual speed against molecules. Now what DOES change is the indicated speed for Vne, since IAS is a function more of HOW MANY molecules are getting rammed into the pitot tube. It's important to know those numbers as you descend out of wave. Vne as marked on the gauge red line is invalid up at the higher altitudes. Flying out west up high it may be more like 110kts indicated when the TAS is actually at redline.

[Duster[_2_]]

Hmmm, I thought Vne Is basically a guess by the FAA that 10% below Vd is safer than 5% below Vd. Vne = 0.9 X Vd (FAR/JAR 23), dive speed Vd determined emperically and is a function of design cruising speed Vc.

[Echo]

Haven't read that, would be an interesting one though. I'd always been under the impression Vne is a flutter protection speed. That's why it's a TAS that doesn't change with weight like Va. Wouldn't be surprised if it was from some old math though.

[Echo]

Also I don't think the FAA is responsible for the manufacturers testing and certification necessarily, especially when they're overseas. I mean, they really had strong oversight in the 737 max....

[Duster[_2_]]

On Saturday, November 9, 2019 at 12:49:14 PM UTC-6, jp wrote:
> I know that Vne can vary with altitude but does it also vary with aircraft weight?

It might be more useful to use Vd rather than Vne, but in any case my guesstimate is that neither Vd or Vne varies with weight (mass), particularly if Vd is determined in a dive with no abrupt control deflections (white arc, right?).

I'm pretty sure calculating Vne employs a surprisingly arbitrary factor chosen to provide a safety margin of 10% below Vd (designed diving speed) which in turn is related to Vc, designed cruising speed. Various references suggest Vd can be either calculated or is the empirically derived maximum speed the test pilot attains before or when the aircraft experiences (1) flutter [Echo has it right I believe], (2) buffeting, (3) other signs of structural stress or (4) nothing [e.g., pilot instructed to go no faster than a pre-determined speed). Vne is a metric apparently not used in certification/testing, but is certainly part of the placard/POH.

Design Cruising Speed (Vc) = 33 * Sqrt (wing loading)
Design Dive Speed (Vd) = 1.4 * Vc
Never Exceed Speed (Vne) = 0.9 * Vd


Disclaimer: take this with a grain of salt, I am not fluent in aerodynamics

[Charlie Quebec]

VNE is certainly tested by the German manufacturers, and part of EASA certification You appear to be using the method applying to powered aircraft in the US.
The flight manual of all Euro gliders clearly states all the operational speed limits.

[jfitch]

On Sunday, November 10, 2019 at 7:02:57 AM UTC-8, Echo wrote:
> Haven't read that, would be an interesting one though. I'd always been under the impression Vne is a flutter protection speed. That's why it's a TAS that doesn't change with weight like Va. Wouldn't be surprised if it was from some old math though.

I always thought Vne was an operational limit to airworthiness, usually derived from the flutter speed. Also that flutter speed was an odd power function of altitude (because it varies somewhat with density but not as fast as IAS, therefore TAS is safe). Also that it is an oscillatory phenomena which depends on the stiffness, mass, and forcing function - so practically does depend on weight (or at least mass), particularly of the wings but secondarily the fuselage. But that function would be way too complicated to put on the placard or even into regulations so no change is assumed.

I also didn't think it was required to air test flutter speed anymore, but could be calculated with mass, stiffness, and CFD codes.

[Duster[_2_]]

On Sunday, November 10, 2019 at 5:49:28 PM UTC-6, Charlie Quebec wrote:
> VNE is certainly tested by the German manufacturers, and part of EASA certification You appear to be using the method applying to powered aircraft in the US.
> The flight manual of all Euro gliders clearly states all the operational speed limits.

Not really; Vd (or Vc) must first be determined before Vne is set. Vne is not discovered by testing (Vne is simply a fixed fraction of Vd to build a safety margin), and of course gliders are flight tested within and beyond the flight envelope. I referenced JAR-22, Joint Aviation Requirements that EASA (including Germany as part of the EU) abides by (JAA responsibilities are/were absorbed by EASA which still follows JARs). Here's an informative German publication : http://journals.sfu.ca/ts/index.php/ts/article/view/193/178

[Eric Greenwell[_4_]]

jfitch wrote on 11/10/2019 5:11 PM:
> On Sunday, November 10, 2019 at 7:02:57 AM UTC-8, Echo wrote:
>> Haven't read that, would be an interesting one though. I'd always been under
>> the impression Vne is a flutter protection speed. That's why it's a TAS that
>> doesn't change with weight like Va. Wouldn't be surprised if it was from
>> some old math though.
>
> I always thought Vne was an operational limit to airworthiness, usually derived
> from the flutter speed. Also that flutter speed was an odd power function of
> altitude (because it varies somewhat with density but not as fast as IAS,
> therefore TAS is safe). Also that it is an oscillatory phenomena which depends
> on the stiffness, mass, and forcing function - so practically does depend on
> weight (or at least mass), particularly of the wings but secondarily the
> fuselage. But that function would be way too complicated to put on the placard
> or even into regulations so no change is assumed.
>
> I also didn't think it was required to air test flutter speed anymore, but
> could be calculated with mass, stiffness, and CFD codes.

Ground vibration testing can also aid in determining flutter characteristics. I
don't know what is required of our gliders. The Perlan 2 glider underwent
substantial amounts of ground vibration testing, and it's flight envelope is
gradually extended by in-flight vibration testing, at various altitudes and
speeds, to corroborate the ground testing results. The vibration is supplied by
eccentric, electric motor driven weights built into the wings.

--
Eric Greenwell - Washington State, USA (change ".netto" to ".us" to email me)
- "A Guide to Self-Launching Sailplane Operation"
https://sites.google.com/site/motorgliders/publications/download-the-guide-1