Vne and where it comes from

[rans6andrew]

Does anyone know whether Vne is derived from some formula and, if it is, what are the parameters that it uses?

The reason I ask is that I have noticed that there is a difference in the Vne figures for the Light Aircraft and Microlight versions of the Foxbat Sport aircraft. As far as I can be sure the only physical difference is the material that the firewall is made from. In the microlight version this is aluminium and the light aircraft uses stainless steel. Otherwise the two aircraft are identical, same engine, same airframe structure, same range of optional equipment etc.

I understand why the light aircraft version has a higher MTOW at 475Kg , the microlight is approved to Section S which calls for a higher G loading but also limits the MTOW to 450Kg. The microlight licence also limits the MTOW of the aircraft the holder can operate.

Can anyone shed any light on this?

Ta,

Rans6....

[Genghis the Engineer]

What are the figures for the two A-22 models?

G

[rans6andrew]

G,

From the dealer spec sheet

Vne microlight 122 mph, group A 130 mph.

also

Max level speed microlight 102 mph, group A 110 mph.

Design dive speed is given as 145 mph for both versions.

with the exception of MTOW all other data given is the same (dimensions, weight, engine details, propeller, flap speeds, stall speed, limit load factors and economy).

The dealer website spec page has the different max level speed figures but not the Vne differences on it. foxbat (http://www.foxbat.co.uk/spec.htm)

Rans6....

[Genghis the Engineer]

It all starts at Vd - the design dive speed, which is based upon structural analysis: the big issue here tends usually to be structural integrity of the wing under high speed (drag and torsion loads). On larger aeroplanes, it may also be based upon flutter onset speed but microlight/VLA class aeroplanes rarely attract that level of analysis.

In flight test, the next speed determined is Vdf, the maximum speed found acceptable in flight test. There are many reasons for Vdf - it may be Vd itself, it may be a point (just below) where stability becomes unacceptably low, some flutter is encountered, the canopy starts to buckle inwards - anything that leaves the Test Pilot feeling that he's about to step over a cliff basically. On a few draggier aeroplanes (most flexwings for example) it's simply the fastest they could get.

Then Vne is normally set to 90% of Vdf; it's not allowed to be set any faster.


So in this case, I'd say its most likely (and I had nothing to do with either programme, so this is an intelligent guess) that something occurred on the microlight version testing to cause the TP(s) to set Vdf at a slightly lower speed than on the group A version. What that was, I've not a clue.

There is also a minimum safety margin between Vh (maximum level flight speed) and Vd. The margin is greater for microlight than VLA-category aeroplanes, which is probably the reason why things have been tweaked to give a lower Vh on the microlight version.

G

[johns7022]

Reminds of the C750 and Premier 1.... long before structural issues became a concern, Vmo was set as a speed where the pilot could still control the left turning tendencies at high speeds.

Reminds also of an Ex-Eastern captain trying to convince me that a Citation 550 could get into 'coffin corner'...I said no....'I have more wing then engine'...I don't think he knew what I was talking about...

[Trim Stab]

Probably just that the two aircraft were certified on different days, by different pilots, under different certification standards.

Determining Vne experimentally is - by its definition - somewhat subjective. Not many TPs of today are prepared to over-speed the aircraft until it suffers some sort of structural failure.

Nowadays, the engineers have an accurate idea of what will cause the limitation - and so certification tests are designed to explore that limitation, rather than actually exceeding it to failure.

For example, if the engineers estimate that wing-flutter will occur at speed X, the TP might perform a series of tests with incrementally increasing airspeed, with an oscillator attached to the wingtip, designed to induce flutter at a lower airspeed than might be encountered in still air. Eg as in this photo:

http://img819.imageshack.us/img819/377/580image228.jpg (http://img819.imageshack.us/i/580image228.jpg/)
Uploaded with ImageShack.us (http://imageshack.us)

So as you can imagine, there is a lot of scope for experimental differences in two different certification procedures.

[johns7022]

Cool pic, thanks..

I wish vibration sensors(and chip detectors) were standard on jets...

Had a bearing go out on a Citation...picked up the vibration before the passengers felt anything, and the shut down was a low impact event...but a vibration sensor(and chip detector) might have detected the failure early, saving LOTS of money on the repairs....

-----

Hat's off to the test pilot guys...seems that is a great job finding those limits and numbers, efficiencies...ect..

[rans6andrew]

Further enguiries with the LAA inform me that the lower Vne/max speed figures are from the LAA having their say and these figures apply to both microlight and light aircraft versions when built in the UK. The higher figures are from the manufacturer and apply to the rest of the world.

Clearly the difference is due to the UK have special/different air to the rest of the world so we need to be restricted further for our own good. As usual.

Rans6....

[Genghis the Engineer]

Further enguiries with the LAA inform me that the lower Vne/max speed figures are from the LAA having their say and these figures apply to both microlight and light aircraft versions when built in the UK. The higher figures are from the manufacturer and apply to the rest of the world.

Clearly the difference is due to the UK have special/different air to the rest of the world so we need to be restricted further for our own good. As usual.

Rans6....

Personally I prefer to think of it as independent and competent aviation professionals doing their jobs properly. (And I had nothing to do with these aeroplanes, so am taking no personal credit here.)

UK has just about the lowest fatal accident rate for light GA in the world, and an enormous fleet of types to choose from. On the whole, I think we're actually doing things about right by requiring at-least one suitably trained and experienced aeronautical professional, who is essentially independent, to have right of veto on new aircraft before people not qualified to form those sort of complex judgements get to fly those aeroplanes.

G

[PPRuNe Pop]

Trim Stab,

Please adjust the size of your pic to fit the size that suits PPRuNe and stops the page being made to sit laterally.

850x850 OR 850x724 is what we require to avoid the issue I have just described.

Thanks.

[Pugilistic Animus]

UK has just about the lowest fatal accident rate for light GA in the world

that's because the UK has no GA, like three folks a year are issued PPLs...part of the health penalty:}


:ouch: