Effect of air density on speeds
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Effect of air density on speeds
Studying for ATPL Performance at the moment and having trouble understanding the effect of air density on speeds. The book says that increased air density increases the values of Vmcg, Vmc and V2, but decreases the values of V1 and VR. I just don't get that at all..........can anyone help?
Vmcg & Vmca: increased air density ==> increased thrust from the engines. If one fails then you will need to be travelling faster to have enough tyre &/or aerodynamic authority to counter the now increased yaw.
V2: Can't remember.
V1: deleted because I was even more of a blithering idiot than usual
VR: Can't remember.
All from memory so I can't guarantee the accuracy!!!!
[ 12 July 2001: Message edited by: Tinstaafl ]
V2: Can't remember.
V1: deleted because I was even more of a blithering idiot than usual
VR: Can't remember.
All from memory so I can't guarantee the accuracy!!!!
[ 12 July 2001: Message edited by: Tinstaafl ]
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Vr (min) is driven by 1.05 Vmca or 1.10 Vmu. If Vmca is reduced, Vr may be also.
V2 is driven by 1.2 Vs or 1.15 (?) Vmca. V2 (min) will reduce (at increasing density altitudes) until is reaches the 1.2 Vs value, and then will remain at that IAS.
V2 is driven by 1.2 Vs or 1.15 (?) Vmca. V2 (min) will reduce (at increasing density altitudes) until is reaches the 1.2 Vs value, and then will remain at that IAS.
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Tee,
You don't indicate what book it is from which you are studying and who might have written it. I note that there are a great many books around which are full of imaginative ideas.
Is the book talking TAS, IAS, CAS, or EAS ? And are we looking at constant TOW ?
To add a little to Tinstaafl's commentary (I might not agree with his V1 comments) and I suspect that Quid might have made a typo or two -
(a) Vmcg tests normally are predicated on no nosewheel steering (ie the nosewheel is rigged to be free to caster to reduce the stabilising effect of nosewheel tyre forces). This allows some margin for contaminated (ie slippery) runway surfaces. The AFM figure is based on the reasonably expected maximum engine thrust. It may be reasonable to talk about the day to day variation in the "real" Vmca/Vmcg as you are suggesting, but the AFM figure is fixed at a reasonably high value. Similarly, the AFM figure for Vmca would normally be fixed.
(b) V2 normally is limited by Vs1 unless, in the lower weight (ie lower Vs1) range, Vmca becomes limiting. So it would appear that the book might be looking at the low speed end of the range. What you are suggesting is not a reasonable statement to make to students without some appropriate clarification. And, in any case, Vmca is declared for reasonably critical conditions so it is generally not going to vary in the Flight Manual.
(c) Vr normally is predicated, within certification boundaries, on a value which, following a failure, will permit the aircraft to end up pretty close to V2 as it goes through 35 feet. I would have expected to see Vr follow whatever V2 does as the main consideration is the rotation rate specified by the manufacturer.
(d) V1, within AFM limits (usually from around, say, 0.85 Vr up to Vr), is not tied to what V2 or Vr are doing and may be varied to suit in the unbalanced field length case. The book probably is talking about a specific presumed situation so we might need to know a bit more about the information it is purporting to give you.
My thought is that one would need to read the book's story in a little more detail (to get the whole story) before talking in excessively sombre tones about its usefulness.
[ 11 July 2001: Message edited by: john_tullamarine ]
You don't indicate what book it is from which you are studying and who might have written it. I note that there are a great many books around which are full of imaginative ideas.
Is the book talking TAS, IAS, CAS, or EAS ? And are we looking at constant TOW ?
To add a little to Tinstaafl's commentary (I might not agree with his V1 comments) and I suspect that Quid might have made a typo or two -
(a) Vmcg tests normally are predicated on no nosewheel steering (ie the nosewheel is rigged to be free to caster to reduce the stabilising effect of nosewheel tyre forces). This allows some margin for contaminated (ie slippery) runway surfaces. The AFM figure is based on the reasonably expected maximum engine thrust. It may be reasonable to talk about the day to day variation in the "real" Vmca/Vmcg as you are suggesting, but the AFM figure is fixed at a reasonably high value. Similarly, the AFM figure for Vmca would normally be fixed.
(b) V2 normally is limited by Vs1 unless, in the lower weight (ie lower Vs1) range, Vmca becomes limiting. So it would appear that the book might be looking at the low speed end of the range. What you are suggesting is not a reasonable statement to make to students without some appropriate clarification. And, in any case, Vmca is declared for reasonably critical conditions so it is generally not going to vary in the Flight Manual.
(c) Vr normally is predicated, within certification boundaries, on a value which, following a failure, will permit the aircraft to end up pretty close to V2 as it goes through 35 feet. I would have expected to see Vr follow whatever V2 does as the main consideration is the rotation rate specified by the manufacturer.
(d) V1, within AFM limits (usually from around, say, 0.85 Vr up to Vr), is not tied to what V2 or Vr are doing and may be varied to suit in the unbalanced field length case. The book probably is talking about a specific presumed situation so we might need to know a bit more about the information it is purporting to give you.
My thought is that one would need to read the book's story in a little more detail (to get the whole story) before talking in excessively sombre tones about its usefulness.
[ 11 July 2001: Message edited by: john_tullamarine ]
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Tinstaafl,
Would not greater acceleration mean you increase V1? - get to a set speed earlier on the runway, allowing more distance for further acceleration before running out of braking distance.
Edited because John T got in first...
O8
[ 11 July 2001: Message edited by: Oktas8 ]
Would not greater acceleration mean you increase V1? - get to a set speed earlier on the runway, allowing more distance for further acceleration before running out of braking distance.
Edited because John T got in first...
O8
[ 11 July 2001: Message edited by: Oktas8 ]
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(Not too fussed about certification numbers in this forum as the gist of the thing is of more use to us as pilots.)
(a) Vmca is usually a single declared figure. I guess that there is no reason why a manufacturer can't schedule different values for different circumstances but this is not the usual approach.
While the "real" day to day Vmca clearly varies (mainly below the declared figure) this is neither quantified nor of much interest to us generally. If one happens to be in circumstances where such matters are critically significant, then one is in more strife than Speed Gordon. The AFM data is based on the certification value. Thus the Vr lower limit as specified in the AFM will not vary from day to day.
(b) V2min relates to Vmca and Vs1. The usual presentation is for V2min to reduce with reducing AUW unless the Vmca consideration becomes limiting at lower weights (mainly on 4 engined aircraft). In the latter case, V2min then becomes constant, reflecting the declared Vmca.
Normally V2 schedules in excess of the V2min selected by the manufacturer are for improved second segment climb gradient reasons.
I don't have my AFM library with me at the moment, but I can't recall any presentation where V2 is predicated on density height ?
Perhaps you could indicate an aircraft for which such variation is the case ?
(a) Vmca is usually a single declared figure. I guess that there is no reason why a manufacturer can't schedule different values for different circumstances but this is not the usual approach.
While the "real" day to day Vmca clearly varies (mainly below the declared figure) this is neither quantified nor of much interest to us generally. If one happens to be in circumstances where such matters are critically significant, then one is in more strife than Speed Gordon. The AFM data is based on the certification value. Thus the Vr lower limit as specified in the AFM will not vary from day to day.
(b) V2min relates to Vmca and Vs1. The usual presentation is for V2min to reduce with reducing AUW unless the Vmca consideration becomes limiting at lower weights (mainly on 4 engined aircraft). In the latter case, V2min then becomes constant, reflecting the declared Vmca.
Normally V2 schedules in excess of the V2min selected by the manufacturer are for improved second segment climb gradient reasons.
I don't have my AFM library with me at the moment, but I can't recall any presentation where V2 is predicated on density height ?
Perhaps you could indicate an aircraft for which such variation is the case ?
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>>>Would not greater acceleration mean you increase V1? - get to a set speed earlier on the runway, allowing more distance for further acceleration before running out of braking distance.
Oktas8,
isn't it the other way round? greater acceleration is most important from V1 to Vr after engine failure at V1, so You can decrease V1, being GO-minded.Higher V1 means You abort T/O at a higher speed, having very little Rwy distance available to continue T/O
Oktas8,
isn't it the other way round? greater acceleration is most important from V1 to Vr after engine failure at V1, so You can decrease V1, being GO-minded.Higher V1 means You abort T/O at a higher speed, having very little Rwy distance available to continue T/O
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JT-
>>I don't have my AFM library with me at the moment, but I can't recall any presentation where V2 is predicated on density height ?
Perhaps you could indicate an aircraft for which such variation is the case ? <<
Sure. My DC-8 is one. We do quite a few operations where the t/o weight is less than max landing weight, so in many cases the *min* speeds are governing.
In the *real* world, most pilots are given simplistic tabulated data to determine V speeds and really don't care (or in some cases, understand) how the numbers were derived. They just use them.
I am responsible for my carrier's 3 engine ferry operations, so the *min* speeds are a very real concern.
Perhaps we don't disagree after all.
>>I don't have my AFM library with me at the moment, but I can't recall any presentation where V2 is predicated on density height ?
Perhaps you could indicate an aircraft for which such variation is the case ? <<
Sure. My DC-8 is one. We do quite a few operations where the t/o weight is less than max landing weight, so in many cases the *min* speeds are governing.
In the *real* world, most pilots are given simplistic tabulated data to determine V speeds and really don't care (or in some cases, understand) how the numbers were derived. They just use them.
I am responsible for my carrier's 3 engine ferry operations, so the *min* speeds are a very real concern.
Perhaps we don't disagree after all.
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quid,
Have never played with DC8s (but wouldn't I like to... would have to be better than even the DC9).
For the benefit of my ignorance in this case, can you explain why the min speeds govern because the TOW is less than MLW ?
For OEI ferry, I can see your interest in minimum speed operations for takeoff ....
Back to the original discussion, does the DC8 schedule varying V2 against density height ?
[ 12 July 2001: Message edited by: john_tullamarine ]
Have never played with DC8s (but wouldn't I like to... would have to be better than even the DC9).
For the benefit of my ignorance in this case, can you explain why the min speeds govern because the TOW is less than MLW ?
For OEI ferry, I can see your interest in minimum speed operations for takeoff ....
Back to the original discussion, does the DC8 schedule varying V2 against density height ?
[ 12 July 2001: Message edited by: john_tullamarine ]
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jt-
My verbal shorthand is what caused some confusion. The MLW in itself is of no signifigance. It is simply that around that weight (and lower) for t/o that the *min* speeds start to come into play. I have made takeoffs as light as 160,000 and as heavy as 320,000. MLW is 240,000 to 270,000 depending on series.
Re: V2 vs. density altitude. If I were more computer literate, I could paste a chart here that will make it very clear. Being unable to do that, I will give a few illustrations:
SL - 15C - 200,000 lbs. V1 Vr V2
Actual speeds 94 111 130
Minimum speeds 120 120 138
3000' -15C - 200,000
actual 97 113 129
min 113 113 130
6000' 15C - 200,000
acual 100 115 128
min 102 102 124
Above this density altitude, the V2 min remains at 124. At the lower density altitudes where V2 min is decreasing with an increase in density altitude, V2 min is limited by Vmca (plus the fudge factor). At around 6000', as the density altitude increases, V2 min remains a constant 124 KIAS. That is where V2 min starts to become limited by the 1.2 Vs speed.
I don't know how this formatting will turn out, but it's the best I can do. This is sure an instance where a picture would be worth 10,000 words. Sorry.
My verbal shorthand is what caused some confusion. The MLW in itself is of no signifigance. It is simply that around that weight (and lower) for t/o that the *min* speeds start to come into play. I have made takeoffs as light as 160,000 and as heavy as 320,000. MLW is 240,000 to 270,000 depending on series.
Re: V2 vs. density altitude. If I were more computer literate, I could paste a chart here that will make it very clear. Being unable to do that, I will give a few illustrations:
SL - 15C - 200,000 lbs. V1 Vr V2
Actual speeds 94 111 130
Minimum speeds 120 120 138
3000' -15C - 200,000
actual 97 113 129
min 113 113 130
6000' 15C - 200,000
acual 100 115 128
min 102 102 124
Above this density altitude, the V2 min remains at 124. At the lower density altitudes where V2 min is decreasing with an increase in density altitude, V2 min is limited by Vmca (plus the fudge factor). At around 6000', as the density altitude increases, V2 min remains a constant 124 KIAS. That is where V2 min starts to become limited by the 1.2 Vs speed.
I don't know how this formatting will turn out, but it's the best I can do. This is sure an instance where a picture would be worth 10,000 words. Sorry.
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Many thanks to everyone who took the time to respond in such detail; an interesting debate which improved my understanding of the subject.
Thanks again
Tee
Thanks again
Tee
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quid,
Thank you ..interesting .. I have some thoughts on the numbers but would need to play with the AFM... not an easy document to come by .. perhaps over a beer one of these days ....
Thank you ..interesting .. I have some thoughts on the numbers but would need to play with the AFM... not an easy document to come by .. perhaps over a beer one of these days ....
