Three Questions

[I-FICO]

Hello everybody...


1)Is there any difference between Vref and Vth.Where can I find definitions?

2)Why it is said that the airplane stalls at the same Ias regardless of the altitude?
I've found some explanations but never really understood.

3) Is there any difference between "Mayday Mayday Mayday" and"Emergency"?

Thank you for your collaboration http://images.ibsrv.net/ibsrv/res/src:www.pprune.org/get/images/smilies/thumbs.gif

[Di_Vosh]

I thought that Vth was the same as Vs (but was for pilots that lithped) :} :} :}

(I'll see myself out...)

[dv8]

LMHO:) :ok:

[grizzled]

Re Mayday and emergency...

The difference is this:

An emergency is a (defined) situation.

MAYDAY is the phraseology used to declare to others that you are in an emergency situation.

One should not transmit the word "Emergency" in place of "Mayday." I can tell you -- from far too many years of experience -- that "Mayday Mayday Mayday" is not only intended to get the attention of others, it DOES. If you have to use those words (God forbid), everyone will shut up and give you their full attention. Then you describe the nature of your emergency.

[Centaurus]

I thought that Vth was the same as Vs (but was for pilots that lithped)

(I'll see myself out...)

Bloody classic. This is precisely why I love reading Pprune:ok:.

[Piltdown Man]

1. I've not heard of Vth but Vref is the threshold crossing speed. Try Googling Vref.

2. For a given weight, an aircraft's lift has to equal it's weight. The molecules of air used to generate lift are the same ones used to show IAS.

3. When you have an Emergency you start your transmission with "Mayday...

PM

[Old Smokey]

1)Is there any difference between Vref and Vth.Where can I find definitions?

Never heard of Vth, perhaps the "th" suggests threshold, in which case more correct terminology would be Vat (Speed at Threshold) or Vapp (Speed for approach). Vapp is typically Vref + Half of the Headwind + All of the gust. Minimum additive to Vref is typically 5 Kt, and Maximum 20 Kt.

2)Why it is said that the airplane stalls at the same Ias regardless of the altitude?

Within limits, an aircraft at any weight will stall at the same Angle of Attack (AoA). The speed varies with the weight. The limits are -

(a) Consistant AoA at stall is fine for low altitude and relatively low speed. At Higher Altitudes and speeds, associated with lower temperatures, increasing Kinematic Viscosity causes the boundary layer to become sluggish leading to seperation of airflow from the wing (stall) at LOWER angles of attack.

(b) For a given weight (mass), again, Consistant Indicated Airspeed (IAS) at stall is fine for low altitude and relatively low speed. Actually, in these constant weight (mass) conditions, stall occurs at a constant Equivalent Airspeed (EAS), but this is extremely close to IAS/CAS in these conditions. At Higher Altitudes and speeds, compressibility causes a widening gap between CAS and EAS, so whilst EAS at stall (below Mcrit) will be constant, CAS will be somewhat HIGHER. With a stall above Mcrit (very high altitudes) Mach Number will govern the speed at stall.

3) Is there any difference between "Mayday Mayday Mayday" and"Emergency"?

Perfectly answered by grizzled, no need to add anything:ok::ok:

Regards,

Old Smokey