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Vne

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Vne

Postby Anonymous » Tue Feb 10, 2015 9:58 pm

Posted by: Peter Knight on 11:00:09 10/27/12 Sat

Every year at permit renewal time I carry out the prescribed flight test including flying the aircraft (J430) at Vne (134knts). It isa non event. The controls tighten up and become more firm and I note that in still air I can probably reach Vne straight and level. I know that other types of aircraft (Europas?) have a Vne that is significantly higher than cruise and the pilots come back as gibbering wrecks after the Vne flight test.

So here's the question...
What fixes the Vne of the Jabiru is it the maximum speed that the engine can sustain in level flight without going over 3300rpm max permitted rpm or is there some mechanical load limitation that fixes this figure. I realised that I had not got the slightest idea where this parameter came from so wondered if anyone knew how it was derived.
Anonymous
 

Re: Vne

Postby Anonymous » Tue Feb 10, 2015 9:58 pm

Posted by: DL on 09:29:21 10/25/12 Thu

http://rgl.faa.gov/Regulatory_and_Guida ... enDocument

Sets out minimum requirements for airframe. As a rule of thumb:

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

Re: Vne

Postby Anonymous » Tue Feb 10, 2015 9:58 pm

Posted by: Peter Knight on 11:00:09 10/27/12 Sat

Many thanks for this. So it comes down to wing loading. I found the following by a Google search. It has a lower density of formulae!

Flight at airspeeds outside the envelope (or at inappropriate speeds in turbulent conditions or when applying inappropriate control loads in a high-speed descent) is risky and can lead to airframe failure. Vne is the IAS which should never be intentionally exceeded in a descent or other manoeuvre and is normally set at 90% of Vd, the 'design diving speed'. For a normal category aircraft, Vd is required to be 1.4 times Vno and, to receive certification, it must be demonstrated, possibly by analytical methods, that the propeller, engine, engine mount, and airframe will be free from overspeeding, severe vibration, buffeting, flutter, control reversal and divergence. To provide some safety margin, Vne is then set at 90% of the lower of Vd or Vdf. Vdf is a diving speed which has been demonstrated without problem in test flights and which must be lower than, or equal to, Vd.

Using the Jabiru data for their J230D (600Kg MTOW) the calculation gives a Vne of 150Knts as against a published Vne of 140Knts. Near enough with a bit of a margin.
Of course the J430 has a higher wing loading (as it has an MTOW of 700Kg which, assuming the allowed g loads are within limits, would give a higher Vne of about 162Knts. I suspect engine overspeed would become an issue well before this figure was reached. Still I think I've learned enough. Thank you for clarifying the design criteria.
Peter
Anonymous
 


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