Variation of VR/V2 with temperature?

[keith williams]

I did download the performance manual. Does it seem strange to you that V2 is lower at higher pressure altitudes, up to 4 knots for the same weight?

It does seem strange, but the author took the figures from the performance manual for a real aircraft, so I have to assume that they are correct.

Any additive for compressibility, although a small factor at this low a speed, would surely increase V2 at higher pressure altitudes.

From what I've seen, V2 has nearly always been exactly 1.2 V stall, simply because manufacturers want the best runway performance, and while V2 can be higher than 1.2 V stall, this would increase runway requirement.

I understand improved climb. However looking at the reference you provided, I see no indication that the example is for that case.

The figures in the V Speed tables are not specifically for the improved climb take-off technique. This subject is dealt with later in the book.


I'm half wondering if position error corrections for the airspeed indicator can provide an explanation, although the only corrections I've seen for jets at this low a speed were a fixed amount, up 2 knot IAS, and regardless of the pressure altitude. It was only once airspeed became greater than 160 knots that position error had pressure altitude corrections applied (as well as the correction depending on airspeed , of course), and temperature was not a factor.



I have never found a definitive explanation of why the figures behave the way they do. My hypothesis goes something like this:

Increasing pressure altitude or temperature tend to:

1.Reduce thrust, which reduces acceleration rate.
2.Increase the TAS : CAS ratio, which increases the acceleration required during take-off.

The engines on this aircraft are flat rated to ISA+15 at MSL, so until this limit is reached, only the increasing TAS:CAS ratio affects the take-off.

The acceleration problem is most critical between observing the engine failure at V1 and reaching V2. So to reduce this problem V1 should be increased and V2 should be decreased. Any significant increase in V1 will require an increase in VR.

For this type of aircraft V2 must be at least 1.13 Vsr and 1.1 Vmca. At the low end of the temperature / altitude scale the constant flat rated thrust will maintain constant Vmca, so the potential to reduce V2 will be restricted. But at higher temperatures and altitudes the reducing thrust will reduce Vmca, making it possible to reduce V2 until it becomes limited by 1.13 Vsr,

This gives an overall sequence of:

1.No changes to V1, VR or V2 while temperatures and altitude are
low.

2.Increasing V1 and Vr after the flat rate limit has been exceeded.

3.Decreasing V2 at higher temperatures and altitudes when the reduced Vmca permits.