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nick murry
29th Jun 2007, 10:51
Morning!

Question guys, see if you can help, I was having a yarn with my crew yesterday..

In a turbo prop a/c what is the limiting factor with the barbers pole? For example in mine it its 250knots (Vne) reducing to .48 mach when climbing (colder temps).

What is the story with .48? is that the speed where the tips of the props start to become inefficient due to the local speed of sound or some other structural limitation?
Thanks in advance

haughtney1
29th Jun 2007, 10:58
First question nick murry is...what type are you flying?

As a general rule of thumb, most modern T/P aircraft...and I don't include the Texas lawn dart, Bandit etc....the .48 mach number will likely relate to the point that the aircraft/airframe is safe to operate with no adverse flight characteristics with a safety margin (typically 10-15%).
At the speeds you are talking about, I suspect it is very unlikely that there are implications as to the prop tips becoming supersonic.
Having said all of this, it is difficult to do nothing more than speculate until we know what aircraft type you are referring too.

nick murry
29th Jun 2007, 11:05
sure,,
B1900

--

haughtney1
29th Jun 2007, 11:19
Then it must be an airframe restriction, as the 1900 is PT6 powered.....

Any current 1900 drivers want to confirm this?

Chuffer Chadley
29th Jun 2007, 11:41
Hello!

If anyone's interested, Vmo is 224kt (below FL200) in the Fokker 50. That works out about .43M in the cruise, although no Mmo is published.

I understand that the limitation for us is structural (to do with the windscreen), rather than anything to do with props.

Cheerio!
CC

Diesel8
29th Jun 2007, 14:28
Like someone else said, it is unlikley that it has to do with the props, they would normally be limited in RPM to keep the tips of the blades from reaching supersonic speed, no real danger in that in itself, but certainly a loss of efficiency.

The limit probably has to do with the airframe structurally, ie windshield, wings or tail. IIRC, I have been told the Dash 8 has one Vmo speed below 14K due to the possibility of birdstrikes on the windshield and another Vmo above 14, but not sure on that one.

I know the Learjet I used to fly, an old 24 had a Vmo of 306, but on later models of the 24 it was 350, this was supposedly due to windshield testing.

411A
29th Jun 2007, 14:30
If you look carefully at the design critiera/limitations for some older (but still later designed) piston transports (Lockheed 1649A Constellation, Douglas DC-7 come to mind), you will find a limiting mach number listed in the AFM.
At first this would seem to be a mistake, however there is a reason, and it has to do with compressibility issues discovered during the early testing of experimental aircraft in the early 1950's.
It was not unusual to find, for example, even where a limiting mach number was not listed, a limitation to indicated airspeeds above a certain altitude, typically in the mid-teens, such as (example)...'reduce Vne by 4 knots per 1000 feet above 14,000.'
Turbine aircraft do not have a Vne as such, they have a Vmo which, if it was a piston transport using the same airframe, would have a Vne considerably higher, as that is how the certification standards were at the time.

Mad (Flt) Scientist
29th Jun 2007, 16:55
On a turboprop type, the Mach number part of the limitation is almost always associated with prop tip speeds.

Simply, Mach numbers of the order of 0.50 are too low to be having an effect on the airframe - interesting stuff really only starts to happen in aerodynamic terms when the Mach number gets to the 0.80-ish values. So there's no expectation of wing, tail, etc having an issue at 0.50.

BUT

The prop tips will be doing something like 0.80 or higher when at high RPM and 0.50-ish aircraft speed. They are the only part of the airframe seeing transonic behaviour at those aircraft speeds. So they are the part almost always first affected by Mach effects, and thus the part which drives any Mach limit.

Mad (Flt) Scientist
29th Jun 2007, 17:08
Also see this reply by Old Smokey (http://www.pprune.org/forums/showpost.php?p=2431507&postcount=7) in a similar thread a while ago, which explains in great detail what's going on for a typical turboprop....

haughtney1
29th Jun 2007, 17:23
I stand corrected...I would never had thought that a PT6 tip turning at 900rpm would be anywhere near sonic type speeds:ok:

Tree
30th Jun 2007, 00:01
On a turboprop type, the Mach number part of the limitation is almost always associated with prop tip speeds.
Simply, Mach numbers of the order of 0.50 are too low to be having an effect on the airframe - interesting stuff really only starts to happen in aerodynamic terms when the Mach number gets to the 0.80-ish values. So there's no expectation of wing, tail, etc having an issue at 0.50.
BUT
The prop tips will be doing something like 0.80 or higher when at high RPM and 0.50-ish aircraft speed. They are the only part of the airframe seeing transonic behaviour at those aircraft speeds. So they are the part almost always first affected by Mach effects, and thus the part which drives any Mach limit.


Very surprising. What about Va, control flutter etc. I have also flown a few of Olive Anne Beech's products (excellent aircraft) and I cannot understand how the prop tips on that type could ever reach transonic speeds. A C-185 with a 3 blade prop in winter is a different story. Are you saying that if I feather the props I can go ahead and do a rapid descent above the Barber (not Barbers) pole safely?

nick murry
1st Jul 2007, 08:27
Thanks guys for the heads up!!

Tree!!
Exactly what our discussion turned to. If you did feather your props and super dived you’re a/c beyond Mmo then technically you should be able to reach speeds all the way up to Vmo (vne) safely? Because, IAS is a measure of dynamic pressure on the structure even at altitudes. So Vmo is a indicated speed not a true speed… is this right??? Me not that smart.

rigpiggy
1st Jul 2007, 11:58
It's a GOAL not a LIMIT.

411A
2nd Jul 2007, 02:46
If you did feather your props and super dived you’re a/c beyond Mmo then technically you should be able to reach speeds all the way up to Vmo (vne) safely?

Ah...no.

Flutter and aircraft structural damage would more then likely ruin your whole day.
Besides, Vmo is likely to be limiting, prior to Mmo.

And, they are not 'goals' they are certificate limits, and as such, are not to be exceeded.

john_tullamarine
2nd Jul 2007, 03:42
More particularly ..

(a) Vmo and Vne are different animals (FAR 23.1505 (http://ecfr.gpoaccess.gov/cgi/t/text/text-idx?c=ecfr&sid=fc7c1dc7eaaffb0c67e330fca8f7dc6c&rgn=div8&view=text&node=14:1.0.1.3.10.7.105.2&idno=14))

(b) Vmo will be limiting at lower pressure heights, Mmo at higher

(c) for Part 23 turboprops, it is common to express the POH limitation in terms of Vmo .. and then adjust Vmo for prop limits (generally at a constant M) rather than call it Mmo explicitly/separately .. eg, I have a KingAir manual to hand and the wordiology is along the lines of "Vmo to FL210 is 260KCAS reducing FL210-FL350 260-192KCAS" The limitation notes that the reducing limit is at 0.58M and there is a reference to the barber pole's reflecting Vmo/Mmo limits.

(d) reference to the FAR and typical POHs will find the usual "These speeds may not be deliberately exceeded in any flight regime" (eg the KingAir manual to hand) or the limitation with the FAR permission "unless a higher speed is authorized for flight test or pilot training " as appropriate to the particular Type. It is understood that unintentional, minor, short term exceedances due minor turbulence (usually) or at the crossover height on descent (if asleep) are part of the normal gameplan.

(e) It's a GOAL not a LIMIT .. quaint view .. I incline to 411A's observation ...

(f) dived you’re [sic] a/c beyond Mmo ... likewise ... in so doing one would be well in excess of the Mmo limit ?

Kiwiguy
4th Jul 2007, 05:06
The 757 has a VMO of just 250 knots below 10,000ft owing to the windshield's ability to withstand birdstrike, however at altitude the VMO is 350 knots and the second aircraft which struck the World Trade centre is calculated to have struck at 466 knots so there is clearly a wide margin between VMO and VNE ?

haughtney1
4th Jul 2007, 12:11
The 757 has a VMO of just 250 knots below 10,000ft owing to the windshield's ability to withstand birdstrike

Its actually 313kts below 8000ft:ok: