View Full Version : Beech 1900 Barbers Pole
11th Mar 2005, 06:21
Been wanting an answer to this question for some time.
The Beech 1900D has a Vmo of 248 KIAS from msl to 13200 feet. Above 13200 feet the Vmo decreases until 195 KIAS at 25000 feet. There is a corresponding barbers pole on the ASI which rests at 248 KIAS at msl and rotates anti-clockwise as the aircraft climbs.
The 1900 C model also has a barbers pole but its Vmo is fixed at 247 KIAS irrespective of altitude.
Why would these aircraft, which only cruise at around 230 - 250 KTAS, require a barbers pole? How could Mach crit be a problem at such low speeds?
Any input would be appreciated. Thanks.
11th Mar 2005, 08:30
Maximum airspeed wether expressed in knots or mach are still the same.Its just another way of describing maximum certified airspeed on. As for Vmo this is also the truth.
You should not however confuse this with "Coffin corner" or (Mach crit) as we are not talking about this. Vmo=Vso
U know with higer altitude the IAS is dropping thus TAS is increasing. therfor the barber pole is going anticlockwise.Showing
maximum IAS which correspond to Vmo.
And not to forget that Beech is trying to sell themselfes with mach numbers in the specs rather than TAS. The sound of it is better I would imagine.
11th Mar 2005, 09:00
I will refrase my question.
Why is there a barbers pole in an aircraft with such low mach speeds? Surely mach crit can't be a problem in that range.
Hope this clarifies matters.
11th Mar 2005, 09:43
Because it can fly at high altitudes, and therfor can easily eceed the maximum IAS in a descend.I only fly the B200 and its certified ceiling is 35000feet, ido not know the ceiling in a B1900 but if this is much lower than the B200 its probably only for practical purposes in the logistics dep.
Forget about mach as I told U its just another way to express speed . The key thing is not to overspeed the A/C thus the barberpole. the higer altitude the lower IAS, even thoug the speed is Higher (TAS/ Mach)
On smaller A/C like a Cessna 172 its no barberpole installed because it will not fly at those great altitudes that will make this a problem. But there is no problem fitting any A/C with this system.
Why we actually use Mach instead of TAS I do not know.:sad:
11th Mar 2005, 09:48
The Barberpole on the 1900 B/C is not fixed it will comeback to around 205 kts @<hidden> FL250 and you will be able to fly 247 when descending through FL 160 give or take some FL's.
With regard to the barberpole there are more things affecting a plane than the Mcrit, coffin corner etc. What about limits as gust factors Va,Vb,Vc from what I recall these are all based on TAS. So theoretically you could fly way to fast at altitude than what the plane was build for. That's why the barberpole comes down.
13th Mar 2005, 18:07
If you look at where your baber pole is above 13,200, you'll find that is is always at exactly Mach 0.48 [MN=TAS/LSS, where LSS= local speed of sound, get the TAS off your GPS, and LSS=39*sqrt(OAT+273)].
Mach 0.48 is listed in the 1900D AFM as the Mmo.
Many textbooks (like Handling the Big Jets, Davies) state that above Mach .5, the prop is subject to huge innefficiencies due to Mach effects at the tip, hence the .48 restriction.
Besides, structural limitations are due to CAS (or EAS if you consider compressiblity effects, which would only give you about 5% error at the top of the 1900D envelope), not TAS.
14th Mar 2005, 11:53
Thanks, makes sense.
What about the 1900"C" though? I think it also has a barbers pole but unlike the "D", the Vmo is fixed at 247 KIAS.
27th Mar 2005, 12:20
The "B & C" models barberpole is not fixed but the POH states 247 KIAS or .47 as VMO/MMO.
27th Mar 2005, 12:44
The JAR/FAR 25 certification has something to do in all that business ? I mean, can a light turboprop like a Piper Cheyenne, using VNE reference, be certified JAR 25 or it has nothing to do with all that ? At first sight I immagine it has absolutely nothing to do, but let's ask :)
28th Apr 2005, 01:39
...is 248 knots. Mmo is 0.48.
FL250 with pax, FL310 with crew only on oxy.
You will find that turbopropellor aircraft, whether FAR23 or FAR25, do not have a Vne...only a Vmo (or in some cases) Mmo as well.
This is primarily due to certification limitations...IE: flutter.
Control surface flutter is very bad news indeed, as a few have found out many years ago.
Believe it or not, even such old types as the DC-7 and (later) models of the Lockheed Constellation had Mach number speed limitations.
Lessons were learned the hard way, many years ago.:sad:
28th Apr 2005, 11:55
Many moons ago when I completed my 1900 ground school by a very experienced guy he mentioned the VNE limit was due to control surface flutter.
So installed on the UB series was small blister on each aileron and the rudder to project a small area into the airflow, when the aircraft was travelling at high speed (ie descent), apparently this helps protect against the onset of flutter with airflow separtion at the trailing edge of the wing.
On the UC, UE series the blisters have been removed from the aileron but still remain on the rudder.
Please feel free to correct me :ok:
Coastrider - are we back at the SAS in KWT
28th Apr 2005, 12:51
As there are a few misconceptions in some of the posts, consider the following which will give you most, but probably not all, of the story for this particular Type ...
(a) the certification bases are given in TCDS A24CE (http://www.airweb.faa.gov/Regulatory_and_Guidance_Library/rgMakeModel.nsf/0/4bd70d173cbc5f4586256fb80048f054/$FILE/A24CE.pdf) at pp 23 for the 1900/1900C and pp 37 for the 1900D. As you will observe, tracking down the specific regs here (http://www.airweb.faa.gov/Regulatory_and_Guidance_Library/rgFAR.nsf/MainFrame?OpenFrameSet) will take more than a few cups of coffee and, as I drink far too much of the stuff already, I'll leave it to you to do some legwork ..
The speed limits appear to be
(i) pp 21: 1900/1900C (C-12J) Vmo = 247 KCAS, Mmo = 0.48
with the changeover at Hp = 12,600 ft
(ii) pp 36: 1900D Vmo = 248 KCAS, Mmo = 0.48
with the changeover at Hp = 13,200 ft
(b) provision is made for the design organisation to use Vmo/Mmo. Current reg for reference (http://www.airweb.faa.gov/Regulatory_and_Guidance_Library/rgFAR.nsf/0/9B920C7B0D15246C852566870071B9D6?OpenDocument) and you are invited to track down any differences for the specific certification.
(c) talking generally, while the operating envelope is a combination of a number of separate requirements resulting in a set of different limits which are overlaid to produce the envelope, in general, one notes
(i) a constant EAS limit at lower levels where dynamic pressures are limiting structurally - Vmo - which needs to account for ASI corrections .. this minor variation may be omitted to keep the limit simpler for operational use.
(ii) as Hp increases, at some point it is usual to see compressibility considerations become limiting and the ASI limit reduces along a constant Mach line - Mmo
If there is no reducing limit at altitude, that is a reasonable indication that either the speed or Hp envelope is not sufficient to see compressibility matters become limiting.