Best angle speed

[dekster73]

If the min. Clean speed ( vzfs) for any aircraft is 210 kts.
Can we fly this IAS all the way up to FL390. ?
No correction needed?
Take in to cosid that the Mach speed ind. Is inop.
Thanks

[westhawk]

No! :confused:

[Jetpipe.]

Hello dekster73,

If i have understood correctly your question is if you can climb with constant IAS/CAS 210kt up to FL390 in clean configuration, without knowing your Mach?

I will asume that no calibration errors exist and therefor IAS=CAS

What happens to all the different speeds when Climbing with constant IAS:
As altitude is increased, EAS decreases. EAS<IAS. Because of IAS being a measure of pressure on the wings (pitot-tube), as altitude is increased the air gets thiner. To counteract that, the aircraft must fly faster to keep a constant Lift. Thats why TAS increases. The increasing TAS though, compresses the air in the pitot more and more, causing an increasing error in the IAS reading. So, EAS + Compr.Error = IAS. Remember, since we are keeping constant IAS and the Compr.Error increases, the EAS must decrease!
Another way to prove the previous said is by analyzing the Lift formula with increased altitude:

Lift = 1/2*ρ*V^2*C*Α

Where: ρ=density, V=TAS, C=Lift coefficient, A=Wing area.

C is constant because we did not change the AoA.
A is constant, clean config.
ρ is decreasing with height.
V must increase to compensate for the density decrease..


The conclusion is that the Compr.Error with 210 IAS constant at FL390 STD is about 9kt so EAS would be around 201kt and that would be the real pressure speed affecting Stall and Clean speeds.
I hope this answers your question!

Jp

ps: For the record your Mach would be around 0.69 so no high speed buffet�

[Desert185]

Theoreticaly speaking, it depends. Don't know why you'd want to do that, but this is how I would do it.

Check low speed buffet speeds at different flight levels on the way up.

Check IAS/Mach equivalents at flight levels nearing level off and cruise IAS for the desired Mach.

During the climb, keep the IAS within the envelope. 210kt may not do it for you.

At cruise, set power for desired Mach, while having high speed buffet, low speed buffet and Mach equivalent IAS either set or written down for reference.

[dekster73]

Maan u sent me back 15 years.
I fly a320, there is no vzfs we have green-dot speed.
It's constant and valuable from ground to 20000 ft then we have to add 1 kt for each 1000 ft.on A320 family g.dot is not constant all the way up ,it increases on the speed tape on PFD after 20000 ft,also VLS increases .
My issue that I didn't have the physical explanation for that !

Q:is EAS is the same concept of ETAS when the cross wind crab is more than 10 degrees?
Thanks a lot for ur help
Brian

[proxus]

....And our T&C are going down because of.......

[de facto]

Dexter,
Did you ever fly on an airbus at fl 390?