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Old 15th Jan 2006, 17:03   #1 (permalink)
 
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Vmo vs MMo

Hey Mates,

Can someone please explain the difference between Vmo and Mmo? I'm thinking that Vmo is a limitation speed based on excessive dynamic pressure from flying at high speeds, while Mmo is a limitation based on shockwave damage from flying at high speeds. Is this correct?

Also, I do not understand why Vmo changes with altitude. I know for piston a/c, Vne does not change with altitude, so it seems logical to me that Vmo would be a constant CAS, while only the TAS would change with altitude.

Thanks.
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Old 15th Jan 2006, 18:17   #2 (permalink)
 
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Re: Vmo vs MMo

Quote:
Originally Posted by Macgyver
Hey Mates,
Can someone please explain the difference between Vmo and Mmo? I'm thinking that Vmo is a limitation speed based on excessive dynamic pressure from flying at high speeds, while Mmo is a limitation based on shockwave damage from flying at high speeds. Is this correct?
Not really.
There are a slew of regulations which must be met within variously defined flight envelopes. The Vmo/Mmo limitations actually applied to an aircraft are essentially a composite of the most restrictive of the various requirements - or the most the manufacturer bothered to show, if smaller.
So, for example, one must show positive directional stability (or non-negative directional stability, if you prefer) within the Vmo/Mmo constrained flight envelope. One may find directional stability problems at high Mach, in which case one option is to simply 'cut back' the speed in ther relevant area. (That's usually cheaper by far than trying to redesign a problem away).
In addition to handling concerns, Vmo/Mmo may arise from loading or aeroelastic concerns, or the need to maintain margins to demonstrated or calculated dive/design speeds.
The difference between the two is indeed the dominance of dynamic pressure or shock characteristics in setting the limit; but it's not "shockwave damage".
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Old 15th Jan 2006, 21:47   #3 (permalink)
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Re: Vmo vs MMo

Vmo variation with altitude reflects EAS/IAS variation. The OEM doesn't have to schedule this and may elect to use a constant, slightly conservative limitation. Not something to worry about unduly.
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Old 18th Jan 2006, 09:51   #4 (permalink)
 
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Some aircraft have a rather strange looking speed envelope. Was wondering myself why. Something like 280kt below 10000' increasing linearly to 320kt at 13000'.
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Old 18th Jan 2006, 13:08   #5 (permalink)
 
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Quote:
Originally Posted by Macgyver
Hey Mates,
Can someone please explain the difference between Vmo and Mmo? I'm thinking that Vmo is a limitation speed based on excessive dynamic pressure from flying at high speeds, while Mmo is a limitation based on shockwave damage from flying at high speeds. Is this correct?
Also, I do not understand why Vmo changes with altitude. I know for piston a/c, Vne does not change with altitude, so it seems logical to me that Vmo would be a constant CAS, while only the TAS would change with altitude.
Thanks.
Vmo is a limitation imposed, as correctly stated by you, to limit excessive Dynamic pressure at higher speeds. As Vmo is expressed as a CAS, and not EAS (which is a direct indication of Dynamic Pressure), the CAS associated with a constant EAS will increase with increasing altitude. A Vmo of, say, 330 CAS (EAS) at Sea Level, would be equivalent to 353 Kt CAS at 30,000 feet. Some manufacturers opt for the variable 'barber pole', some opt for the simpler constant CAS. For the example given, 330 CAS at 30,000 feet is only 311 Kt EAS, a much lower dynamic pressure.

I would not go so far as to say that Mmo is imposed to prevent shockwave damage, but rather to avoid, or contain within reasonable limits the CONTROL problems of excessive Mach number.

wondering, at first glance, this seems like a bird strike limitation. Umm, birds don't fly above 10,000 feet do they?

Regards,

Old Smokey
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Old 18th Jan 2006, 13:24   #6 (permalink)
 
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Yep, bird strike protection seems to be the obvious. But, then again check out those high altitude flyers: http://magazine.audubon.org/birds/birds0011.html
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Old 19th Jan 2006, 08:36   #7 (permalink)
 
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VMO is a structural limitation ( not to broke a part of the airframe, say for ex. the windshield) and is increasing with altitude, and MMO is an aerodynamic limitation (not to get supersonic on wings) and does nor vary.
Hope it helps!
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Old 19th Jan 2006, 11:12   #8 (permalink)
 
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We have types where Mmo does vary with altitude; it is not a given that it be fixed. Reasons include PE accuracy and handling characteristics which (both) vary with alt.
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Old 19th Jan 2006, 16:03   #9 (permalink)
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<<wondering, at first glance, this seems like a bird strike limitation. Umm, birds don't fly above 10,000 feet do they?>>

Well if it wasn't a bird, I'd love to know what I hit in a 737 north of Naples at 250kts/11,000'! I recall from long ago, a 727 (Andes?) hit a large bird which came in through the radome and might have killed the copilot- certainly messed up the cockpit, and that was up at 30,000'+
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Old 19th Jan 2006, 20:06   #10 (permalink)
 
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speed bumps?

G'day wondering!
What do you want the speed profile to look like? Rough Q & A.
35ft above the deck and CAS 175, what will it be at 1500ft above and cleaned up? Still about 175? Suppose you then accelerate in level flight to CAS 250 and then climb? You reach 10000ft and CAS is still 250 but Mach has risen to 0.45 and TAS to 290. Yes? Suppose you then accelerate in level flight to say CAS 275? Here you lose me a bit. I think you'd still climb at a fixed CAS say 275 but Mach No and TAS would still be increasing. Yes or no?[You wouldn't alter speed again until say M0.82 but that's a long way above you yet].
Flight path angle is directly related to (Thrust-Drag)/Weight. Horizontal and vertical speeds are directly related through flight path angle. CAS, TAS and Mach No are related by formulae. The options to plot profile in terms of CAS, TAS etc against time, distance, height, altitude, Flight Level are your choice but which part looks strange? There will be coggles when you accelerate and change from one regime to another will there not? Won't they be mutually explainable?
OS! Bird strikes do take place above 10000ft. On a Boeing data recorder they are dead ahead and show up as momentary speed reductions. On an Airbus they are from dead aft and show up as momentary speed increases. This explains otherwise unexplainable speed profile variations.
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Old 20th Jan 2006, 08:47   #11 (permalink)
 
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Oh my gawd! I'll have to find the appropriate symbols on the new PPRuNe layout. My reference to "birds don't fly above 10,000 feet do they?" was tongue in cheek, a dig at those manufacturers who impose a "bird" limitation only below 10,000 feet. I think that (hypoxic?) birds have been encountered well into the 30 thousands!

Regards,

Old Smokey
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Old 26th Jan 2006, 17:29   #12 (permalink)
 
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Question

@ enicalyth,

I reckon we are not talking about the same thing. I was not refering to a take off/climb profile but to airspeed limitations e.g. VMO changes between 10000' and 13000' fom 280kts to 320kts. I donīt think this makes much sense from a purely aerodynamic point of view.
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Old 27th Jan 2006, 14:24   #13 (permalink)
 
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No wondering, it doesn't make any sense from a purely aerodynamic point of view. To make any sense of this situation, it's necessary to know -

(1) What are the bird strike considerations, and, more importantly,

(2) What is the aircraft type?

Regards,

Old Smokey
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Old 27th Jan 2006, 18:49   #14 (permalink)
 
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Hmmm... My understanding is that Vmo can be influenced by bird strike certification standards. 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).

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. 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. Th net result is that Vmo becomes limited by FAR 25.571 (Bird Strike) or FAR 525.333 & 335 whichever is more limiting.

Just my opinion based on reading very complex inter-linked FARS by a humble pilot.

Richard
A320 Captain and bird strike researcher
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Old 27th Jan 2006, 22:18   #15 (permalink)
 
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Quote:
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.

Quote:
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.
Quote:
Originally Posted by §25.1505
VMO/MMO must be established so that it is not greater than the design cruising speed VC
Quote:
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:
Quote:
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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Old 28th Jan 2006, 01:01   #16 (permalink)
 
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Mad (Flt) Scientist:

Thanks for taking the time to explain in detail the sections of the additional FAR's. I had read these FAR's and believed I had a basic understanding of how they should be applied, but did not feel confident in communicating an interpretation in a clear and concise manner.

Thanks also for bringing in the earlier version of FAR 25.571 - I plead no excuse other than forgetfullness.

As usual another day has gone by and I have learned something.

Richard
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