Variation of VR/V2 with temperature?
 

Hi all,

I am going through some passed papers for a performance exam and would like some help understanding the reasoning behind the following question.

For the same pressure altitude how does VR/V2 change with an increase in temperature?

I have looked everywhere and discussed it with various collegues and we can't seem to agree on an answer.

Thanks in advance.

I would imagine there would be an increase in the speeds with an increase in temperature.

Rise in temperature gives drop in pressure and density. Vr is based on Vmu, Vmu is to do with lift generated by the wings at max AOA on the ground so I would think it will increase with an increase in temp.

Just my simple understanding.

TAS icreases with increase in temperature.
So for a given VR, your speed across the Air mass will be higher.
Reason why perf using assumed higher temperature is a conservative method in the STOP (v1) and the GO (V2) decision.

The speed increment between Vr and V2 in terms of TAS is proportional to thrust minus drag. Below the flat-rated temperature thrust is constant and V2-Vr is approximately constant in terms of TAS, but reduces with increasing temperature in terms of IAS.

Above the flat-rated temperature thrust reduces with temperature and hence acceleration. V2-Vr then reduces with increasing temperature in terms of TAS as well as in IAS.

Spanglish, I think you got a detailed info alright:E

In our takeoff performance software LPC, all the speeds remain the same all the way to the flex temp.

MD83FO;

When using 'flexible' thrust settings (except when limited by Vmc which is determined for the actual temperature), speeds and distances are determined at the assumed temperature, which is conservative for lower temperatures.

your increase in temp is going to reduce air density, so effectively your density alt. If we look at a massive temp increase. Then an airfield with an elev of 2,000` and an outside temp of 40 deg C, is going to perform as if it was at a much higher altitutde. get it?
Less dense. More forwards speed required (G/S) to achieve the same airspeed (TAS)
therefore need a longer runway. Have you got a longer runway? If not, either you don`t go, or you unload cargo, why would you do that? Or aska few passengers to disembark ( hint,start with the cargo first . . ) you may consider removing some fuel, not an option that most, if not all pilots tend to entertain, although physically possible, you will then have to change you flight routing and alternates etc. etc., etc. - figure out your speed adjustment from all that and it should come easy now. ok? Good luck with it all.

Spanglish asks "For the same pressure altitude how does VR/V2 change with an increase in temperature?"

If you mean for a particular take off weight, then for any jet I've flown, Vr and V2 remain the same, regardless of any change in temperature.

Are we overlooking the obvious here?

As temperature increases, the Limiting Takeoff Weight (or Regulated Takeoff Weight) reduces. That's normal for pretty much every aircraft flying. Reduced weights require reduced speeds. Thus, V1, Vr, and V2 will all reduce.

I rest my case your honour.

Best Regards,

Old Smokey

El correcto

Smokey,

the OP made no mention of limiting the take off weight, just to keep the PA the same and vary the temperature. He made no reference to the situation of maximum allowed take off weight.

Guys, Keep it simple and correct:

1) V1 is Thrust related so depends on your density and temperature (thus also your flex. tmp. calculations)

2) Vr and V2 are always MASS related and are the same for a certain Take-off Mass. They do not change unless your payload or fuel changes.

3) Your MTOM depends Runway length and condition (think contamination or up/downslope), ambient temperature and density.

It's actually an interesting question, if you take a look at airbus' published INDICATED Vr and V2 speeds they decrease as temp increases but you're being fooled:

E.g.
This is directly out of the airbus A320 T/O quick reference charts:

As an example for a Config 1+F T/O from a 3000m runway @ PA:0ft:

@-20 degrees: Vr: 162KIAS (KTAS:152) V2:164KIAS (KTAS:154)

@ 50 degrees: Vr: 150KIAS (KTAS:159) V2: 152KIAS (KTAS:161)

So the Vr and V2 INDICATED speeds (the ones published on the chart) DECREASE as temperature increases but when you convert them to TAS you'll find the opposite: the TRUE speed INCREASE as temp increase.

Don't let the indicated temperatures fool you, the aircraft needs a higher TAS through the air to fly in warmer air.

Airmann,

Are these different Vr and V2 speeds for the same take off weight?

No the max t/o weights are obviously different. But nevertheless, the higher TAS are for a lower max T/O weight. One would have to look at TAS over IAS. If anything IAS means nothing in terms of aerodynamics, it's just what the pilot is looking at, and gives him a reference by which to fly the aircraft.

Don't be fooled...
 

Airmann,

If the take off weight changes, then of course V2 changes.

However, for the same weights, do V2 and Vr change as pressure altitude and/or temperature changes? You have the data?

Does MD83FO have it right, way back in post 6?

No hawk, it's a little difficult to do a comparison of weights because of the way airbus have set up their charts. When we calculate t/o speeds and flex temp for given conditions, we always use the speeds computed for a given runway length, pressure alt and temp, regardless of our weight because we assume the highest permissible RTOW and use those speeds, the speeds would never change, only the flex temp would. Yes MD83FO has it right.

For example with the numbers I have used above at -20 degrees the max RTOW is 83.8. And the calculated V speeds are based off the assumption of a 3000 meter runway, -20 degrees and 0ft PA and 83.8t. Now suppose the aircraft was only loaded to 70t TOW. We would find that the max ambient temp at which a 70t a/c could takeoff given the runway length and PA. In this case it would be ~67 degrees. So we would use a flex of 67 but the V speeds would not change. If on the other hand we had a a/c loaded to 77t the max temp given the runway and PA is ~51. So we would use flex 51 BUT THE SAME SPEEDS AS BEFORE.

Now I understand this is a quirk of Airbus as they base their speeds off the max RTOW, so all aircraft using those numbers are safe, If its any help the max RTOW in the first instance (-20 degrees) is 83.8t, whereas the max RTOW in the second instance is 77.6t (50 degrees). So even with a LOWER TOW (77.6t) the V speeds when expressed in TAS are higher due to the lower temp. The temperature is dictating the speeds in this instance not the weights.


BTW this is not to say weight doesn't have an effect, it's just that airbus don't let it by always assuming the max permissible T/O weight. I think that if someone had charts from something smaller, a piper or Cessna we could find the answer.

This question comes up in 2 different past papers for the HK CAD performance exam and the supposed right answer is..

VR increases, V2 decreases.

Thanks for your input folks.

Spanglish

It is possible that the HK Authorities are using JAR/EASA based material.

If you go to the UK CAA website and search for CAP 698 you will find the performance manual that is used for the JAR/EASA performance exams. This book is based on an early version BOEING 737 with engine flat rated to ISA +15 degrees Celsius.

If you open the PDF and go into pages 65, 66 and 67 you will find tables for the V1, VR and V2. These tables are based on balanced take-offs, with no stopway or clearway.

On page 65 you will see a table that allocates letters A, B, C, D, E or F for increases in temperature or pressure altitude.

On pages 66 and 67 you will find tables for various conbinations of temperature/PA, weights and flap settings.

If you select one of these tables and pick a fixed weight, then move from left to right across the you will see how V1, VR and V2 vary with increasing temperature or pressure altitude for this aircraft undertaking this type of take-off.

In many cases there is no change until you get to the high temperatures.