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Old 27th Jan 2006, 22:18
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Mad (Flt) Scientist
 
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Originally Posted by Canuckbirdstrike
The certification standard from FAR 25.571 is "Impact with a 4 lb bird at Vc (design Cruise Speed) at seal level or 0.85Vc at 8,000 feet whichever is more crtitical". This is for a FAR 25 aircraft windshield or airframe, for the empennage the speed is the same but the bird weight is 8 lbs (due to the loss of a Viscount in 1962). The design requirement is "The airplane must be capable of successfully completeing a flight during which likely structural damage occurs" (doesn't say how challenging that successfu landing might be and whether you get to use the airplae again in the near future).
Just a note that that requirement applies to aircraft to which amendment 25-96 (Apr. 28, 1998) is applicable; the previous version of the rule (Amdt. 25-86, 61 FR 5222, Feb. 9, 1996) does not provide relief at 8,000ft, requiring 1.0Vc at all altitudes specified. So depending on when your specific type was certified and to which standard might influence whether birdstrike is shaping the Vmo limitation.

Originally Posted by Canuckbirdstrike
My research shows that Vc is a function of Vd (Design Dive Speed) and Vb (Design Speed for Maximum Gust Intensity). All the details including a diagram are defined in FAR 525.335 with an explanation of the manoeuvring envelope contained in FAR 525.333. Reading in the text there is a reference that states that Vc cannot be greater than 0.8 Vd. Vmo is required to be less than Vd.
True, but more importantly, per FAR §25.1505, Vmo may not be greater than Vc.
Originally Posted by §25.1505
VMO/MMO must be established so that it is not greater than the design cruising speed VC
Originally Posted by Canuckbirdstrike
Based on my understanding, if an aircraft fails the FAR 25.571 test at 280 knots then following the trail through the the FARS would cause Vmo to be limited to some value less than 280 knots. The net result is that Vmo becomes limited by FAR 25.571 (Bird Strike) or FAR 525.333 & 335 whichever is more limiting.
§25.1505 is the key to understanding setting Vmo. It establishes a number of restrictions:

1. not greater than the design cruising speed VC
2. sufficiently below VD/MD or VDF/MDF, to make it highly improbable that the latter speeds will be inadvertently exceeded in operations
3. The speed margin between VMO/MMO and VD/MD or VDF/MDF may not be less than that determined under §25.335(b)
4. The speed margin between VMO/MMO and VD/MD or VDF/MDF may not be less than that found necessary during the flight tests conducted under §25.253

1. is going to be driven by "how high can we push Vc" for the various structural design cases, including birdstrike, but also possibly including gust loads. §25.341(a)(5)(i) specifies gust requirements as a function of altitude; if you see Vmo shifts near these altitude breakpoints it may indicate a gust loading limitation:
Originally Posted by §25.341(a)(5)(i)
Positive and negative gusts with reference gust velocities of 56.0 ft/sec EAS must be considered at sea level. The reference gust velocity may be reduced linearly from 56.0 ft/sec EAS at sea level to 44.0 ft/sec EAS at 15000 feet. The reference gust velocity may be further reduced linearly from 44.0 ft/sec EAS at 15000 feet to 26.0 ft/sec EAS at 50000 feet.
2, 3, and 4 are driven by "how high can we push VD" with loading (and flutter) concerns taken into account, and also "how high can we (or did we) push VDF" PLUS the aircraft upset/out-of-trim speed increase behaviour per the relevant requirements in e.g. § 25.253 and § 25.255
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