Turboprop Mmo

[Virtus]

A Metro 3 has a Mmo of 0.52 - why is it so low? What would the limiting factor be?

Thanks!

[pattern_is_full]

Mmo is not always tied to classic aerodynamic Mach effects (wing shock waves, etc.)

It may also be limited by structural strength (windscreen resistance to air flow or bird strikes) or by micro-Mach problems (shock waves over rivets, that aren't an immediate significant problem in themselves, but may cause vibration, leading to accelerated metal fatigue).

Mmo(perating) is just that - an operating limit. In many cases it may be a fairly significant limit like Vne - but it can also simply be like a highway speed limit (yes, you can safely go faster - but don't do it!)

But, like you, I'd be glad to hear from MIII pilots/engineers who know of the specific reason(s) for the Metros' limit.

[thetimesreader84]

I would imagine its more to do with prop tip speeds approaching Mach 1?

[westhawk]

Or perhaps an engine inlet/propeller imposed limitation. The inlets for the TPE 331 engines are situated very close to the propeller blade roots. Seems to me that the airflow to the engine inlet might be the first mach imposed limitation reached. Maybe one of the original design team will be along to verify? :)

westhawk

[Virtus]

It may also be limited by structural strength (windscreen resistance to air flow or bird strikes)

I would imagine its more to do with prop tip speeds approaching Mach 1?

I would imagine that resistance to airflow would be an IAS limit and bird strikes would be a TAS limit.

The Vmo is 246 KIAS which transitions to a Mmo of 0.52 at 17,800' under ISA conditions. Mmo is maintained until the max operating pressure altitude of 31,000'. Max TAS will be at 17,800' and decrease up to 31,000'. Since max prop tip speed is based on TAS, the max tip speed (TAS) happens at 17,800'. At 17,800' the helical tip speed is Mach 0.874 - at 31,000' the helical tip speed is Mach 0.907. The calculations take into account the local speed of sound at each altitude. Therefore, it doesn't make sense that the Mmo limit of 0.52 would be directly related to propeller tip speed and it would not be related to a bird strike limit as the TAS decreases with altitude when following a constant Mach number.

Mmo(perating) is just that - an operating limit. In many cases it may be a fairly significant limit like Vne

Just as a technical note, Vmo is related to Vc which is related to Vno. That is probably why you don't normally see a yellow range on the ASI of an airplane limited by Vmo. Vmo shouldn't be exceeded but it's not as serious as exceeding Vne (related to Vd).

That's how I understand the regulations - if someone involved in testing airplanes could correct me on that or confirm it, that would be great.

The inlets for the TPE 331 engines are situated very close to the propeller blade roots. Seems to me that the airflow to the engine inlet might be the first mach imposed limitation reached.

I hadn't really thought too much about the inlet. I understand that the airflow entering can't be above Mach 1.

http://www.nationalflight.com/images/Turbine/engine-cutout-big.jpg

In that image the inlet looks like it gets smaller which would increase airspeed before it contacts the first compressor stage. However, the image is a bit misleading - it doesn't quite show how the intake is carved out. The air comes in and wraps around the center shaft that connects the engine to the gearbox and contacts the entire surface of the compressor face. Without the exact specs it's hard to say if it's narrowing, staying the same, or expanding. Having looked in there in real life, I don't think it changes much. It would have to almost double the speed of the air to get it to hit Mach 1 - I don't see that happening.

Regarding the interference caused by the lower part of the blade - I'm not sure how that would affect it.

[pattern_is_full]

It could just be that Vmo/Mmo for the MIII is simply a result of FAA paperwork and math.

I don't know if the MIII is certified under Part 23 or Part 25 - but the critical phrasing involving definition of Vmo/Mmo is the same for both - and note that it is a derived speed, defined simply as being "sufficiently below VD /MD (design diving speeds)...to make it highly improbable that the latter speeds will be inadvertently exceeded in operations.":
_______________

FAR Part 23

§ 23.1505 Airspeed limitations.
(a) The never-exceed speed V NE must be established so that it is—

(1) Not less than 0.9 times the minimum value of V D allowed under § 23.335; and

(2) Not more than the lesser of—

(i) 0.9 V D established under § 23.335; or

(ii) 0.9 times the maximum speed shown under § 23.251.

(b) The maximum structural cruising speed V NO must be established so that it is—

(1) Not less than the minimum value of V C allowed under § 23.335; and

(2) Not more than the lesser of—

(i) V C established under § 23.335; or

(ii) 0.89 V NE established under paragraph (a) of this section.

(c)(1) Paragraphs (a) and (b) of this section do not apply to turbine airplanes or to airplanes for which a design diving speed VD /MD is established under § 23.335(b)(4). For those airplanes, a maximum operating limit speed (VMO /MMO airspeed or Mach number, whichever is critical at a particular altitude) must be established as a speed that may not be deliberately exceeded in any regime of flight (climb, cruise, or descent) unless a higher speed is authorized for flight test or pilot training operations.

(2) VMO /MMO must be established so that it is not greater than the design cruising speed VC /MC and so that it is sufficiently below VD /MD , or VDF /MDF for jets, and the maximum speed shown under § 23.251 to make it highly improbable that the latter speeds will be inadvertently exceeded in operations.
______

I.E. there is no particular reason (aerodynamically) to set the Mmo at 0.52 - except to mathematically make it "sufficiently below" VD/MD to prevent inadvertent exceeding of those speeds.

What's "sufficiently below?" Who knows? 5%? 15%? Plug in an educated engineering guess.

[Virtus]

I.E. there is no particular reason (aerodynamically) to set the Mmo at 0.52 - except to mathematically make it "sufficiently below" VD/MD to prevent inadvertent exceeding of those speeds.I find it hard to believe that an engineer would randomly select a Mach number for a certification criterion.

I'm thinking the 0.52 limit might have to do with vibration and buffeting, and/or stability. Increasing the AoA from level, 1G, flight will increase the speed of the air over the top of the wing, which is already going faster than the free stream speed. Depending on a few factors, you can get control force reversal as you increase the load factor - which usually results in an UNSAT for that exercise.