barber pole and why does is move?

[RB211-524G]

ok on the plane i fly, SW4, and i'm sure others as well, the barber poll automatically decreaces the Vne/Mmo above approx 17,800ft. on the Sw3, the calculation is done manually ie in your head 2kts/1000ft. my question is why does the Vne/Mmo need to be decreased at all, is it a limiting speed in relation to the propeller and the speed of sound (my personal theory) or does it have something to do with structrual integrity of the a/c. I have already discounted the idea that it is some sort of limiting critical mach # on the wing as the Mmo of the Sw4 is .52 which is still very much within subsonic flight for everything except the propellor, mayby. ok hope this makes sense, i look forward to hearing some responses, thanks :ok:

[john_tullamarine]

Suggest you run a search as the topic comes up regularly.

At the lower levels, Vmo will be limiting and you may see the IAS limit increase a little as you climb reflecting the EAS. Limit often relates to wing skin structure loading .. occasionally one reads of D-cell destruction in significant exceedances, for instance. At crossover, the Mmo takes over and the IAS reduces with further climb to bring the TAS back in line with local sonic speed.

The actual limiting design case for a given aircraft will not necessarily be obvious.

[Old Smokey]

Dammit john_tullamarine, a person goes on leave, finds an Internet cafe, and you grab all the good ones:*

J_T is right (as usual) about the Barber Pole increasing at lower levels due to the EAS limit, which is a structural consideration. With increasing altitude, the CAS for a given EAS will increase.

Mmo will be imposed at higher levels due to the fastest moving part of the aircraft reaching critical mach numbers (Note that I said critical Mach numbers, not Mcrit, some exceedance of Mcrit can be tolerated). For the Propeller aircraft, the fastest moving part is the propeller tip, and this will be the first part of the aircraft to reach critical Mach numbers. Thus, even though you may not have a Mach Meter, the Barber Pole will reduce with increasing height to observe the Mmo.

Even much older aircraft like the F27 had one Vno/Vne up to 20000 feet, and then another up to 25000 feet. If you did a quick comparison, they were the same Mach Number, a propeller induced Mmo limitation.

Regards,

Old Smokey

[john_tullamarine]

.. sorry about that, mate ... and, if you are on leave, why aren't you down this way to have a beer or three ?