V1/Vr/V2 due to temperature and weight

Find or share aviation knowledge
new reply
User avatar
ilya1502
Staff Sergeant
Posts: 290
Joined: 06 Nov 2013, 08:46

V1/Vr/V2 due to temperature and weight

Post by ilya1502 »

There are so knowledgeable and experienced pilots in this community, that I think it is the right place to ask.

There are several explanations why speeds behave as they do including several in this thread. But here is a page from the EMB-120 flight manual.

In this table, speeds generally tend to decrease with temperature and altitude for low weights. But why do they behave differently for higher weights?

Image

User avatar
AKar
A2A Master Mechanic
Posts: 5207
Joined: 26 May 2013, 05:03

Re: V1/Vr/V2 due to temperature and weight

Post by AKar »

Do you mean higher OATs?

Generally speaking, in case of balanced field, you make TODR = ASDA.

When very hot and heavy, in this case, you can think that the V1 is pushed up by required single-engine performance in case of continued takeoff instead of being pushed down by ASDA in case of rejected takeoff (the TODR increases as well with temperature and weight). Hence, higher V1 (the minimum continued takeoff speed in this case) coming with the longer takeoff distance required. These figures read out so that your reject threshold rises as you are unable to climb out if you continued your takeoff with one engine unless at high enough speed. Effectively, you increase the required runway length to increase your minimum single engine continuing speed, the V1.

If I misunderstood, please note. :)

-Esa

User avatar
ilya1502
Staff Sergeant
Posts: 290
Joined: 06 Nov 2013, 08:46

Re: V1/Vr/V2 due to temperature and weight

Post by ilya1502 »

Esa,

Thanks for the reply. What you are describing is quite obvious, what is not — why then speeds do the opposite for light weights? I would expect TODR to increase with temperature for any given weight. However, for 8000 kg speeds are way higher for colder OATs.

User avatar
AKar
A2A Master Mechanic
Posts: 5207
Joined: 26 May 2013, 05:03

Re: V1/Vr/V2 due to temperature and weight

Post by AKar »

TODR is not given here, note that the various weights listed imply different TODA/TODR here, from a separate table.

If you are speaking of V2, it is a certifying speed selected (and demonstrated) by the applicant to be the target speed to be reached at the screen height after a continued takeoff in case of an engine failure at or after V1. Here I presume the higher performance at cold temperatures has resulted in higher target speeds, 118 KIAS being apparently the kind of real deal. Higher speed may provide better climb performance and better controllability, hence, if the screen height is retained, it probably is preferable.

-Esa

User avatar
ilya1502
Staff Sergeant
Posts: 290
Joined: 06 Nov 2013, 08:46

Re: V1/Vr/V2 due to temperature and weight

Post by ilya1502 »

Okay, but what about Vr then?
For 8000 kg it is 110 kias at -54C while 100 kias at +50. At the same time, for 12000 kg it is 112 kias at -54C and 121 kias at +50. For a heavy airplane it increases with OAT, but for a lighter one it decreases with it.

User avatar
AKar
A2A Master Mechanic
Posts: 5207
Joined: 26 May 2013, 05:03

Re: V1/Vr/V2 due to temperature and weight

Post by AKar »

Yes, I see what you're talking about.

Generally speaking, the VR goes hand-in-hand with V2. However, the effect of the temperature is the opposite: VR usually goes up with the temperature (to counteract reduced lift at lower density), while V2 normally goes down with increasing temperature (to counteract worse acceleration capability). I am not sure why they have chosen such figures here, but I presume it has to do with the requirement to reach the screen height at V2 speed within takeoff distance in case of engine out (or with a distance margin of 15 % with all engines operating). In shaded figures, they even have selected V2's lower than VR's.... which, yes, you can do I presume. But technically this would mean that you'd slow down slightly from the rotation to the climb segment.

Do you happen to have more pages of this manual? In particular, I'd want to check if we are VMCG/VMCA -limited here.

I may want to ask if someone I know who has been flying various turboprops has ran into anything like this, the V2 lower than VR.

-Esa

User avatar
ilya1502
Staff Sergeant
Posts: 290
Joined: 06 Nov 2013, 08:46

Re: V1/Vr/V2 due to temperature and weight

Post by ilya1502 »

Thanks, Esa. Unfortunately, it's the only page I have.

Please, ask, if you can, it will be interesting to know.

jester292
Airman Basic
Posts: 9
Joined: 18 Apr 2020, 20:29

Re: V1/Vr/V2 due to temperature and weight

Post by jester292 »

AKar wrote: 20 Oct 2019, 03:18 Yes, I see what you're talking about.

Generally speaking, the VR goes hand-in-hand with V2. However, the effect of the temperature is the opposite: VR usually goes up with the temperature (to counteract reduced lift at lower density), while V2 normally goes down with increasing temperature (to counteract worse acceleration capability). I am not sure why they have chosen such figures here, but I presume it has to do with the requirement to reach the screen height at V2 speed within takeoff distance in case of engine out (or with a distance margin of 15 % with all engines operating). In shaded figures, they even have selected V2's lower than VR's.... which, yes, you can do I presume. But technically this would mean that you'd slow down slightly from the rotation to the climb segment.

Do you happen to have more pages of this manual? In particular, I'd want to check if we are VMCG/VMCA -limited here.

I may want to ask if someone I know who has been flying various turboprops has ran into anything like this, the V2 lower than VR.

-Esa
Hi Esa,

Regarding the takeoff performance and calculations for V1, Vr, and V2: V2 can never be less than Vr. There are times they will be equal, only because V2 can't be less than Vr. Also, Vr cannot be less than V1. They can in some instances be equal, just like Vr and V2. There is a caveat, and that is with windshear or improved climb scenarios where we delay rotation until Vr+20. Vr is still less than or equal to V2, but we delay the rotation up to Vr+20 (unless we start running out of pavement!). I haven't dived into the "why do the higher weights affect V1, Vr, and V2 differently than lower weights", but I can elaborate more later. Off to work!
A2A P-40
A2A P-47

User avatar
AKar
A2A Master Mechanic
Posts: 5207
Joined: 26 May 2013, 05:03

Re: V1/Vr/V2 due to temperature and weight

Post by AKar »

jester292 wrote: 24 Apr 2020, 02:35 Hi Esa,

Regarding the takeoff performance and calculations for V1, Vr, and V2: V2 can never be less than Vr. There are times they will be equal, only because V2 can't be less than Vr. Also, Vr cannot be less than V1. They can in some instances be equal, just like Vr and V2. There is a caveat, and that is with windshear or improved climb scenarios where we delay rotation until Vr+20. Vr is still less than or equal to V2, but we delay the rotation up to Vr+20 (unless we start running out of pavement!). I haven't dived into the "why do the higher weights affect V1, Vr, and V2 differently than lower weights", but I can elaborate more later. Off to work!
Hey Jester,

Yes, I came back to this later on (but didn't find a moment to post and then forgot about it!) when I checked how CS-25 defined the criteria for V2. V2MIN can be less than VR, but V2 is required, with some other things, to be no less than "VR plus the speed increment attained (in accordance with CS 25.111(c)(2)) before reaching a height of 11 m (35 ft) above the takeoff surface". To be honest, I've never before seen any circumstances where V2 speeds of less than VR speeds were suggested, so I did not have a recollection of this specific requirement.

However, EMB-120 nowadays would probably be certified under CS-23 (commuter category). Under that this exercise is a bit more interesting, albeit of no practical significance to be honest. Yet, the aircraft could, under some conditions, have its VR restricted from below either by being no less than 1.05 VMC or no less than 1.10 VSI. It could be selected to be higher, though, with no explicit higher limit if I recall, actual performance limitations aside. The V2, on the other hand, could be selected to be restricted to not be less than 1.10 VMC or less than 1.20 VSI. Assuming this criteria still allows the airplane to actually reach the V2 before reaching height of 11 m, the V2 could, as it appears, on paper, actually be less than VR, as it is not explicitly forced to be equal or higher.

I am not sure about ruling against which the EMB-120 was actually certified. Also, I am not a flight performance engineer, so there probably are several yea-buts that I miss, and, of course, I understand that this is a pure paper exercise with little to no practical significance. :)

-Esa

User avatar
CAPFlyer
A2A Aviation Consultant
Posts: 2241
Joined: 03 Mar 2008, 12:06
Location: Wichita Falls, Texas, USA

Re: V1/Vr/V2 due to temperature and weight

Post by CAPFlyer »

AKar wrote: 24 Apr 2020, 09:19I am not sure about ruling against which the EMB-120 was actually certified. Also, I am not a flight performance engineer, so there probably are several yea-buts that I miss, and, of course, I understand that this is a pure paper exercise with little to no practical significance. :)
The first of those "yea buts" is that it's an EMB-120.... :lol:

Don't know the EASA certification information on the EMB-120 (a search of the EASA TCDS database gets no results interestingly, only EMBRAER aircraft in the database are the Phenoms), but the US Type Certificate says the Certification Basis was FAR 21 (CERTIFICATION PROCEDURES FOR PRODUCTS AND ARTICLES ), FAR 25 (AIRWORTHINESS STANDARDS: TRANSPORT CATEGORY AIRPLANES), and FAR 36 (NOISE STANDARDS: AIRCRAFT TYPE AND AIRWORTHINESS CERTIFICATION).

The one thing I know about Turboprops is that the engines factor significantly more into the V-Speed and Balanced Field Length calculations than jets. Between the issues with P-Factor, spiral airflow, and potential for adverse yaw and critical engine, so some of the oddities with V-Speeds probably has to do with that, especially when it comes to propeller performance in extreme conditions. The Hamilton Standard props were also subject to some extreme scrutiny and I'm sure that had an effect as well - http://aerossurance.com/safety-manageme ... re-emb120/

Would love to see the rest of an EMB-120 takeoff performance chart set too.
Image

User avatar
AKar
A2A Master Mechanic
Posts: 5207
Joined: 26 May 2013, 05:03

Re: V1/Vr/V2 due to temperature and weight

Post by AKar »

Yeah, indeed that's one of the disclaimers I didn't add. :) I only mentioned EASA CS as those are the ones I've once gone through and have worked in reference to (albeit they are certainly changed since in bits and pieces). They are mostly similar to the FAA equivalents but not exactly.

Indeed I did not check this (I should have!), but the ANAC TCDS [pdf] tells the EMB-120 is certified as a transport category aircraft, not as a commuter like I assumed. Hence, the applicable certification specification under EASA would be CS-25. That's probably a one good reason why the figures where the V2 speeds are higher than VR speeds are shaded, for those would not be 'legal' or 'applicable'. Anyways, this is a very interesting case for those being included at all, and really the only example of me seeing this as far as I can remember.

-Esa

new reply

Return to “Flight Academy”

Who is online

Users browsing this forum: No registered users and 6 guests