VREF compared to what?
 

I'm havinga a bit of trouble figuring out all the comparisons for VREF according to JAR25?

Is it:
VREF = 1,23 x VSRO (vsro meaning what?)
VREF = 1,3 x VSO (Vstall)
VREF = ? x VS1
VREF = ? x VMCA

Anyone who knows?

Vref should be calculated with reference to the stalling speed in the landing configuration, so to my mind it's 1.3xVso. I haven't heard of Vsro so I can't completely discount that I'm afraid, someone else will clear it up I'm sure but that'd be my initial guess.

Vref is not ‘referenced’ to something; it is a reference speed for landing performance and is published by manufactures as a reference for the pilot to use during the approach.
CS/JAR/FAR 25.125
VREF may not be less than –
(i) 1.23 VSR0;
(ii) VMCL established under CS25.149(f); and
(iii) A speed that provides the manoeuvring caability specified in CS25.143(g).

CS-25 ammendment 1. via EASA

Additionally, as long as the various minima are met (which will vary according to when and where a type was certified) it can be any LARGER number the OEM felt a good idea. Usually desire for short landing distances dictates as low a Vref as can be managed, but sometimes other factors come into play.

Vsr0 is 'reference' stall speed i.e. stall speed determined according to the 1'g' stall rules. Take away the 'r' for 'reference' and you get Vs0, the old-fashioned 'Vmin' stall speed. You should never see both the 1.23Vsr0 and the 1.3Vs0 rules at the same time, because you only determine stall speed the one way or the other, and then use the relevant speed factors for the method you used.

Can you elaborate on the different rules between 1.23Vsr0 and 1.3Vs0? I'm a bit confused, because it seems that the safety margin is being reduced. Isn't Vs0 at 1g as well?

Vso, I seem to recall, is the "minimum" speed in the stalling maneouvre (power off), which may be well below the speed at which the aircraft first starts to stall.

Has it been determined that if a manufacturer were to determine both 1.3 Vs0 and 1.23 Vsr0, would these two speeds now be about the same, for conventional aircraft types?

Perhaps MFS can provide some more insight.

Hawk

Hawk37's explanation is basically it; Vs0 is the minimum speed demonstrated in a stall, and the aircraft can actually be below 1'g' during the manoeuvre with the result that the aircraft wouldn't actually be capable of sustained level flight at Vs0. Consequently it's sometimes called "Vs-min"

Vsr0 is the minimum speed corrected to 1'g' flight - which means each speed is corrected for the instantaneous 'g' and THEN you take the minimum - or, alternatively, you can work out the lift coefficients during the manoeuvre, then take the maximum, then calculate Vsr0, because it's basically the same thing. The correction to 1'g' is why this is sometimes called "Vs1g".

The reason the regs have changed to the Vs1g method is that its inherently more consistent - depending on how you fly a stall you can get different Vsmin values, while Vs1g is more analytically defined. It's much harder to 'cheat' (intentionally or inadvertently) at Vs1g! However, a Vs1g speed is invariable higher than a Vsmin speed FOR THE SAME MANOEUVRE because the 'g' was invariably below 1.0. Therefore had the same 1.20/1.30 factors been retained for V2 and Vref, these speeds would have increased, with a consequent performance (and economic) penalty; since those factors are arbitrary, the argument was made (and its logical) that the old factors, combined with the Vs-min method, gave a "safe" V2 and Vref, and therefore the goal should be to end up at similar speeds with the new method - so-called "equivalent safety".

It was determined that for then-existing aircraft that a Vs1g speed would be about 6% higher than a Vs-min speed, so the factors were all reduced accordingly - so-called "reduced reference speed (factors)" - 1.20 became 1.13, 1.30 became 1.23, etc. For any given aircraft one method or the other might give 'better' V2 or Vref numbers; the main point is that the Vsr0 number is consistent.

(Incidentally, there were minimum 'g' levels for a valid Vs-min stall speed - you couldn't fly a 0.5'g' 'bunt' and claim that as stall speed, but values down at 0.97-0.95'g' were certainly seen in some cases)